Sex-specific differences in nutrient regulation in a capital breeding caterpillar, Spodoptera litura (Fabricius)

Nutrient requirements by male and female insects are likely to differ, but relatively little is known regarding how sexes differ in their regulation of macronutrient acquisition. The present study reports the results from a laboratory experiment in which behavioural and physiological components of nutrient regulation were compared between male and female caterpillars of Spodoptera litura (Fabricius). When provided with choices between two nutritionally complementary foods (one is a protein-biased diet and the other a carbohydrate-biased diet), both males and females adjusted their food selection to defend an intake target. However, the composition of diet preferred by the two differed, with females selecting significantly more protein than males with no difference in carbohydrate intake between the two. When confined to single diets with varying mixtures of protein and carbohydrate [P:C ratios, expressed as the percentage of diet by dry mass: protein 42%:carbohydrate 0% (p42:c0), p35:c7, p28:c14, p21:c21, p14:c28, p7:c35], females consumed more macronutrients than did males across on all P:C diets except the extremely carbohydrate-biased diet (p7:c35). Under both choice and no-choice feeding condition, such sex differences in nutrient intake were not expressed until late in the feeding stage of the final stadium. Sexes also differed in post-ingestive utilization of ingested nutrients. Females utilized ingested protein for body growth with greater efficiency compared to males, presumably reflecting provisioning their adult needs for protein to develop eggs, whereas males were more efficient at depositing lipids from carbohydrate intake than females.     

A correlation between macronutrient balancing and insect host-plant range: evidence from the specialist caterpillar Spodoptera exempta (Walker)

In an earlier study, we showed that the ingestive responses of the generalist caterpillar Spodoptera littoralis to foods imbalanced in their protein:carbohydrate content is similar to generalist locusts, but differs from that of specialist-feeding locusts. Here we further pursued the comparison by repeating the experiments using a closely related specialist caterpillar, Spodoptera exempta. First, caterpillars were allowed to self-compose a diet of preferred protein:carbohydrate balance by mixing between nutritionally complementary foods. Then, they were confined to one of five imbalanced foods, in which we measured the trade-off between over- and under-ingesting the two nutrients. On complementary foods, the caterpillars actively regulated their protein and carbohydrate intake. In the no-choice experiment, those fed excess-protein foods ingested small surpluses of protein compared with generalist feeders, thus showing a pattern of nutrient balancing similar to that observed in specialist locusts. Utilisation data indicated that ingested excesses and deficits were to some extent offset by differential utilisation. Evidence also showed that post-ingestive responses of the specialist S. exempta were less flexible than those observed in the generalist S. littoralis, a pattern which is again in accordance with comparisons of acridids differing in their host-plant range.     

Nutrient utilization by caterpillars of the generalist beet armyworm, Spodoptera exigua

Abstract.  Beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), caterpillars are destructive crop pests responsible for considerable annual economic losses. These generalist herbivores are challenged with a diversity of dietary choices that can impact their survival, development and fecundity. In the present study, dietary choices of S. exigua caterpillars, based on the protein to digestible carbohydrate (P : C) ratio of the food, and the impact of nutritionally unbalanced foods on caterpillar performance are assessed. In choice experiments, individual third-instar caterpillars are offered simultaneously a P-biased and a C-biased food until pupation. Caterpillars feed nonrandomly and select a slightly P-biased diet (22P : 20C). In no-choice experiments, second instar caterpillars are reared until pupation on diets ranging in P : C ratio from extremely P- to extremely C-biased. High mortality and delayed development are observed on the C-rich, P-poor diets, highlighting the potential deleterious effects of excess carbohydrates and the importance of protein for growth and development. Diet-dependent differences in pupal weight or pupal lipid reserves are not observed. This contrasts with closely-related Spodoptera species where pupal mass and lipid stores increase on C-rich, P-poor diets. On the extremely P-biased diet, performance is similar to that of individuals reared on the self-selected diet, suggesting that these caterpillars may efficiently be deaminating excess amino acids to generate carbon skeletons, which are shunted into lipid biosynthesis. Spodoptera exigua caterpillars exhibit flexible and efficient pre-ingestive nutrient intake regulation and post-ingestive utilization, allowing these generalist feeders to cope with the heterogeneous diets they may encounter.     

Do food protein and carbohydrate content influence the pattern of feeding and the tendency to explore of forest tent caterpillars?

This study examines whether the ratio of protein to carbohydrate affects the timing of meals and the propensity to explore of forest tent caterpillars (Malacosoma disstria). The behavior of fourth instar caterpillars was observed on three semi-defined artificial diets varying in protein (p)-carbohydrate (c) ratio. These diets were (a) p14:c28, (b) p28:c14, and (c) p35:c7. The probability of initiating feeding at first contact with the food and the duration of the first feeding event did not vary across diets, suggesting not much difference in phagostimulatory power. There was also no difference in the total time spent eating, at rest and in motion between diets. However, the timing and duration of meals varied significantly; more short meals were observed on the carbohydrate-biased diet. The duration of pauses between meals also increased with food protein content. Furthermore, caterpillars on the carbohydrate-biased diet were more likely to leave the trail leading to the known food source and to discover a second food source, suggesting that protein deprivation promotes exploration. These findings shed insight into the physiological responses to protein and carbohydrate ingestion and demonstrate how post-ingestive effects can favor consumption of foods containing protein without invoking an explicit mechanism of independent nutrient regulation, but simply by influencing the pattern of feeding and the propensity to explore.     

Macronutrient regulation in the Rasberry crazy ant (<Emphasis Type="Italic">Nylanderia</Emphasis> sp. nr. <Emphasis Type="Italic">pubens</Emphasis>)

Animals grow and optimize performance when they collect foods in amounts and ratios that best meet their species-specific nutritional requirements. For eusocial organisms like ants, where only a small fraction of the colony members collect food, increasing evidence demonstrates that strong macronutrient regulation occurs at the colony level. Here, we explored regulation of protein and carbohydrate (p:c) intake in the Rasberry crazy ant, Nylanderia sp. nr. puben. We did this using dry artificial foods (14–42% total macronutrient content) having a range of fixed p:c ratios in a series of choice and no-choice laboratory experiments and used worker mortality to gauge colony-level costs associated with active nutrient regulation. Choice experiments revealed that colonies preferred carbohydrate-rich foods and self-selected a diet having a p:c ratio ~1:2. No-choice experiments demonstrated that food p:c ratio only moderately affected worker food collection behavior, likely because colonies regulated the intake of only the non-limiting nutrients. Absolute worker mortality was generally high, but lowest in colonies feeding on the food having a p:c ratio of 1:2 (the p:c ratio ants self-selected in the choice experiment), although mortality was not significantly affected by food p:c ratio. The self-selected p:c ratio in our study is consistent with that observed in other recent ant nutrient regulation studies. Taken together, the results from these combined studies reveal emerging commonalities among ants in macronutrient regulation strategies, and similarities in foraging behaviors and costs associated with macronutrient regulation. Finally, from a methodological perspective, the high mortality observed in our study, when compared with other recent studies, suggests that ant nutrient regulation studies should be conducted using foods having high moisture and total macronutrient content.     

Influence of dietary nutritional composition on caterpillar salivary enzyme activity

Caterpillars are faced with nutritional challenges when feeding on plants. In addition to harmful secondary metabolites and protein- and water-limitations, tissues may be carbohydrate-rich which may attenuate optimal caterpillar performance. Therefore, caterpillars have multiple strategies to cope with surplus carbohydrates. In this study, we raise the possibility of a pre-ingestive mechanism to metabolically deal with excess dietary sugars. Many Noctuid caterpillars secrete the labial salivary enzyme glucose oxidase (GOX), which oxidizes glucose to hydrogen peroxide and gluconate, a nutritionally unavailable carbohydrate to the insect. Beet armyworm, Spodoptera exigua, larvae were restricted to diets varying in protein to digestible carbohydrate (P:C) ratio (42p:21c; 33p:30c; 21p:42c) and total nutrient concentration (42% and 63%). High mortality and longer developmental time were observed when caterpillars were reared on the C-biased, P-poor diet (21p:42c). As the carbohydrate content of the diet increased, caterpillars egested excess glucose and a diet-dependent difference in assimilated carbohydrates and pupal biomass was not observed, even though caterpillars restricted to the C-biased diet (21p:42c) accumulated greater pupal lipid reserves. Larval labial salivary GOX activity was also diet-dependent and gluconate, the product of GOX activity, was detected in the frass. Unexpectedly, GOX activity was strongly and positively correlated with dietary protein content.     

Temperature‐by‐nutrient interactions affecting growth rate in an insect ectotherm

Temperature and nutrition are two prominent environmental variables influencing juvenile growth rate in ectotherms. These two factors interact in complex ways. Here, we present a comprehensive analysis of the interactive effects of temperature and nutrition on various components of fitness (growth rate, survival), food intake, and level of energy storage in an insect herbivore, caterpillars of Spodoptera exigua Hübner (Lepidoptera: Noctuidae). In a factorial experimental design, final‐instar caterpillars (i.e., fifth instars) were individually reared at one of three constant temperatures (18, 26, and 34 °C), in which they received one of six diets differing in their ratio of protein and digestible carbohydrate [P:C mixture, expressed as the percentage of diet by dry mass: protein 42%:carbohydrate 0% (42:0), 35:7, 28:14, 21:21, 14:28, and 7:35]. Within the range of test temperatures, larval growth rate increased with rising temperature and was strongly affected by P:C mixture, reaching a maximum on moderate P:C diets at each temperature and falling at very high and low P:C mixtures. There was a significant temperature*diet interaction, such that the difference in growth rates between temperatures was greatest on moderate P:C diets and least on the most extreme diets (42:0 and 7:35). Food intake rate patterns followed a similar trend to growth rate. Rapidly growing animals at high ambient temperature suffered high mortality across all dietary P:C mixtures, but to a greater extent on the extremely unbalanced diets. This suggests that there are developmental and physiological costs associated with fast growth at high temperature, as indicated by high rate of pupation failure and reduced lipid storage efficiency. Our study shows how temperature and nutrition interplay to mediate phenotypic variations in growth rates and energy utilization in an insect ectotherm.     

The effects of nutritional imbalance on compensatory feeding for cellulose-mediated dietary dilution in a generalist caterpillar

Abstract. The interactive effects of macronutrient balance [protein (P) : carbohydrate (C) ratio] and dietary dilution by cellulose on nutritional regulation and performance were investigated in the generalist caterpillar Spodoptera littoralis (Boisduval). Caterpillars were reared through the final stadium on one of 20 foods varying factorially in macronutrient content (P + C%: 42, 33.6. 25.2 or 16.8%) and P : C ratio (5 : 1, 2 : 1, 1 : 1, 1 : 2 or 1 : 5). The animals compensate by eating more of diluted foods, but suffer reduced nutrient intake in proportion to the degree of dilution. Increase in food intake with dilution is greater on balanced than imbalanced foods and this is reflected in greater reduction of dry pupal mass with dilution in the latter. Whereas dilution results in a reduction in the amount of whichever macronutrient is in excess in the food, by contrast, the ability to compensate for the deficient macronutrient in the food is unaffected by nutrient imbalance. Excess protein intake due to nutritional imbalance (diets with high P : C ratios) results in a regulatory decrease in the efficiency of retention of ingested nitrogen relative to restricted protein intake on oppositely imbalanced foods (low P : C ratios). By contrast, decreased protein intake due to dietary dilution is associated with a non‐regulatory reduction in the efficiency of retention, irrespective of P : C ratio. Dilution is similarly associated with reduced utilization efficiency of ingested carbohydrate. The ecological implications of these results are discussed.     

Nutritional regulation in nymphs of the German cockroach,Blattella germanica

Synthetic foods varying in protein-carbohydrate ratio and total nutrient concentration were used in food selection experiments to investigate the ingestive and post-ingestive regulation of macronutrients by male and female nymphs of the German cockroach, Blattella germanica (L.) (Dictyoptera: Blattellidae). For both nutrient imbalance and food dilution, food ingestion varied between treatments with the effect that nutrient ingestion was regulated. However, this mechanism was insufficient to compensate for some food dilution treatments. In those cases, the regulation of protein intake was prioritised over that of carbohydrate intake, and two additional regulatory responses were seen. Firstly, cellulose digestion supplemented shortfalls in dietary soluble carbohydrates, and secondly the feeding period within the stadium was prolonged. These ingestive, post-ingestive and developmental responses were orchestrated in such a way that, in all treatments, nutrient gain approached similar levels, despite the variation in food properties.     

Comparison of tree genotypic diversity and species diversity effects on different guilds of insect herbivores

Although the effects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity (e.g. intra‐ and interspecific), most studies have analyzed these effects separately. We compared the effects of genotypic diversity of big‐leaf mahogany Swietenia macrophylla with that of tree species diversity on two specialist caterpillars (Hypsipyla grandella stem borers and Phyllocnistis meliacella leaf miners) and three generalist leafhoppers (Cicadellidae) feeding on mahogany in a large‐scale (7.2 ha) forest diversity experiment in southern Mexico. The experiment consisted of fifty‐nine 21 × 21‐m plots, with 64 tree saplings each (3‐m spacing between plants). Plots were either mahogany monocultures or species polycultures of four species (including mahogany) and – within each of these two plot types – mahogany was represented by either one or four genotypes. Throughout a five‐month period, beginning six months after planting, we measured mahogany growth and monitored herbivore and predator (spider) abundance. We found no effect of mahogany genotypic diversity on either specialist caterpillars or generalist leafhoppers, and this result was consistent across levels of tree species diversity. In contrast, species diversity had significant effects on both specialists but neither of the generalist herbivores. Specifically, species diversity lowered H. grandella attack at the middle of the sampling season, but increased attack at the end of the season, whereas P. meliacella abundance was consistently reduced. Such effects were not mediated by effects of species diversity on plant growth (of which there were none), but rather through resource heterogeneity. Diversity did not influence spider abundance. This study is one of few to directly compare sources of plant diversity, and uniquely compares such effects among herbivores with contrasting life histories (e.g. diet breadths). Overall, we demonstrate that plant species diversity effects outweigh those of genotypes, and our results suggest that such effects are stronger on specialist than generalist herbivores.     

Food mixing strategies in the desert locust: effects of phase, distance between foods, and food nutrient content

Food mixing strategies were compared in the cryptically coloured, relatively sedentary `solitarious' and the highly mobile, conspicuously coloured `gregarious' phases of the desert locust, Schistocerca gregaria. Based on phase related differences in behaviour and nutritional regulatory responses, we predicted that solitarious nymphs, compared to gregarious nymphs, would move less between nutritionally complementary foods, particularly as the distance between the foods increased. We manipulated the nutritional composition [protein (p) and digestible carbohydrate (c) content] of two foods in an experimental arena and varied the distance between the foods using a factorial experimental design. Results indicated that in general, solitarious nymphs showed greater fidelity to individual food dishes than did gregarious insects (i.e., they concentrated their feeding mainly on one dish). However, results also demonstrated that for both phases fidelity to a particular food dish increased as the distance between the dishes increased, and that the number of switches between dishes decreased with increasing distance. In the smallest arenas, though, gregarious nymphs switched more frequently between the two food dishes than solitarious nymphs, even when the two dishes contained the same, near-optimal food (p18:c24). When challenged by having the two dishes either placed furthest apart (2 m) or more divergent in nutritional composition (p29:c13 vs. p7:c35), insects of both phases regulated protein intake more strongly than carbohydrate intake, by eating more from the dish containing higher-protein food.     

Combined effects of allelochemical-fed and scarce prey on the generalist insect predator Podisusmaculiventris

1. The simultaneous effects of allelochemicals ingested by herbivorous insect prey and prey scarcity on the performance of a generalist insect predator were examined.
2. Fifth-instar predatory stinkbugs ( Podisus maculiventris : Pentatomidae) were fed caterpillars ( Manduca sexta : Sphingidae) in three prey scarcity treatments: every day (unlimited amount), one caterpillar every third day, one caterpillar every fifth day. The caterpillars were fed either a plain diet or a diet containing rutin, chlorogenic acid and tomatine, which are three of the major allelochemicals in tomato leaves ( Lycopersicon esculentum : Solanaceae), the preferred food of these caterpillars.
3. Food consumed, efficiency of conversion of ingested food to biomass (ECI), biomass gained, stadium duration and relative growth rate (RGR) of predators were negatively affected by prey scarcity. The allelochemicals negatively affected food consumed and ECI.
4. There were prey scarcity by allelochemical interactions for ECI, biomass gained and RGR. For ECI, the allelochemicals had a greater negative impact on the predatory stinkbugs when prey were scarce. When prey diet contained allelochemicals, biomass gained and RGR declined more steeply with increased prey scarcity. There was an allelochemical by predator gender interaction for biomass gained. Allelochemicals had no effect on biomass gained by female stinkbugs, whereas biomass gained declined more steeply with increased prey scarcity for male stinkbugs fed caterpillars containing allelochemicals than for males fed control caterpillars.     

Dietary self‐selection and rules of compromise by fifth‐instar Vanessa cardui

We examined dietary self‐selection and rules of compromise for protein (P) and digestible carbohydrate (C) intake by fifth‐instar Vanessa cardui L. (Lepidoptera: Nymphalidae: Nymphalini). We presented six fat‐free diet pairs to larvae in a choice trial to determine the ‘intake target’. In addition, we fed larvae seven fat‐free single diets differing in dietary nutrient ratio in no‐choice trials to determine the rules of compromise they exhibit when constrained to a singular, sub‐optimal dietary source. In choice trials, caterpillars regulated nutrient intake to a ratio of 1 protein to 1.09 carbohydrate (1P:1.09C), exhibiting tighter regulation of protein than of carbohydrate. Furthermore, larvae from different diet pair treatments did not differ in pupal mass or stadium duration. In no‐choice experiments, larvae reduced consumption on increasingly protein‐biased diets and increased consumption on increasingly carbohydrate‐biased diets, relative to a 1P:1C ratio diet. Differences in carbohydrate consumption were much greater between no‐choice treatments than differences in protein consumption. Dietary nutrient ratio affected pupal mass when accounting for initial larval mass. Pupal mass decreased as nutrient ratio was shifted off of 1P:1C, but to a greater extent when the ratio was skewed toward carbohydrate. Stadium duration increased as nutrient ratio diverged from 1P:1C, being more pronounced when shifted toward carbohydrate than toward protein. Regulation to near 1P:1C is consistent with results found for other Lepidoptera, and the rule of compromise exhibited by V. cardui is consistent with that expected for a generalist herbivore.     

Three hundred and fifty generations of extreme food specialisation: testing predictions of nutritional ecology

We used a strain of diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), that had been reared for approximately 350 generations in a precisely characterised environment to test hypotheses regarding the influence of nutritional heterogeneity on the evolution of nutrient regulatory responses. Caterpillars were maintained with ad libitum access to a diet that emulated that of an extreme nutritional specialist, comprising a homogeneous food of fixed nutrient composition. We measured performance (survival, development rate, and pupal mass), as well as the protein and carbohydrate intake of individual caterpillars confined to one of a range of single foods differing in their protein, carbohydrate, and water content. In a separate experiment, we measured the amount and balance of protein and carbohydrate self-selected by caterpillars presented with nutritionally complementary foods. Results showed a close fit with three of four predictions about the nutritional responses of 'nutrient specialist' feeders: (1) survival, development rate, and pupal mass were highest for animals given diets with the protein:carbohydrate composition of the ancestral culture diet, and dropped off sharply with higher and lower protein:carbohydrate balance, (2) caterpillars coped poorly with dietary dilution by water, irrespective of the macronutrient balance, and (3) the self-selected intake point corresponded with the macronutrient balance that gave peak performance (i.e., that of the ancestral culture diet). The fourth prediction, that caterpillars would be disinclined to over-ingest nutrients on imbalanced diets, was at best weakly met. We hypothesise that the evolution and maintenance of the specialist strategy might, paradoxically, require some degree of environmental heterogeneity.     

Seasonality directs contrasting food collection behavior and nutrient regulation strategies in ants

Long-lived animals, including social insects, often display seasonal shifts in foraging behavior. Foraging is ultimately a nutrient consumption exercise, but the effect of seasonality per se on changes in foraging behavior, particularly as it relates to nutrient regulation, is poorly understood. Here, we show that field-collected fire ant colonies, returned to the laboratory and maintained under identical photoperiod, temperature, and humidity regimes, and presented with experimental foods that had different protein (p) to carbohydrate (c) ratios, practice summer- and fall-specific foraging behaviors with respect to protein-carbohydrate regulation. Summer colonies increased the amount of food collected as the p:c ratio of their food became increasingly imbalanced, but fall colonies collected similar amounts of food regardless of the p:c ratio of their food. Choice experiments revealed that feeding was non-random, and that both fall and summer ants preferred carbohydrate-biased food. However, ants rarely ate all the food they collected, and their cached or discarded food always contained little carbohydrate relative to protein. From a nutrient regulation strategy, ants consumed most of the carbohydrate they collected, but regulated protein consumption to a similar level, regardless of season. We suggest that varied seasonal food collection behaviors and nutrient regulation strategies may be an adaptation that allows long-lived animals to meet current and future nutrient demands when nutrient-rich foods are abundant (e.g. spring and summer), and to conserve energy and be metabolically more efficient when nutritionally balanced foods are less abundant.     

Generalist caterpillar prey are more palatable than specialists for the generalist predator Iridomyrmex humilis

Summary Experiments are described which test the hypothesis that more host-specific species of caterpillars should be less aceptable to a generalist predator than polyphagous species. Caterpillars of all species were tested in paired choice tests with the Argentine ant, Iridomyrmex humilis. Experiments were replicated ten times, videotaped and later analyzed. Brightly colored specialist species (normally considered to be aposematic) were clearly the least palatable, while more cryptic specialists were also significantly less acceptable than generalists overall. Leaf-tying species were considered separately; all were highly palatable independent of host range. The results indicate that among caterpillars that do not construct leaf shelters, those with a wide post range are more acceptable than those with a narrow host range. This is consistent with the notion that generalist predators provide selection pressure favoring narrow host range in their herbivorous prey.     

Foliage‐age mixing within balsam fir increases the fitness of a generalist caterpillar

1. Manipulative field studies were carried out to evaluate the foliage age preference–performance relationship for an extreme generalist herbivore, the whitemarked tussock moth (Orygia leucostigma Smith) (Lepidoptera: Lymantriidae), within balsam fir [Abies balsamea (L.) Mill]. 2. Field surveys indicated that early instar caterpillars fed almost exclusively on young (i.e. current‐year) foliage, whereas late instars caterpillars fed on both young and mature (i.e. 1‐ and 2‐year‐old) foliage. 3. Survival of early instar caterpillars was highest in treatments where current‐year and/or 1‐year old foliage were available, but decreased significantly on older foliage. In contrast, late instar caterpillars had the highest survival when allowed to feed on all age classes of foliage, whereas potential fecundity was highest for late instars that fed on young foliage. 4. Overall, caterpillars had 32–65% higher fitness when able to feed on all rather than just one age class of foliage. 5. These results support both the ‘complementary diet' hypothesis, which states that dietary mixing of different‐aged foliage can increase nutrient uptake and/or dilute harmful secondary plant chemicals, and the ‘ontogeny’ hypothesis, which attributes changes in diet to changes in the nutritional needs and/or tolerance to plant defences of juvenile insects as they develop.     

Molecular heterogeneity of pro-atrial natriuretic factor

Analysis by two-dimensional gel electrophoresis and Western blotting of the atrial natriuretic factor (ANF) content of atrial granules revealed the presence of at least 15 immunoreactive spots whose molecular mass distribution ranged from 16.8 to 35 kDa and their pI values from 5.12 to 5.98. About 90% of the immunoreactive ANF material was contained within four spots (spot 1: 34.8 kDa, pI 5.67; spot 5: 16.8 kDa, pI 5.50; spot 6: 16.8 kDa, pI 5.67; spot 7: 16.8 kDa, pI 5.98). Investigation of the molecular nature of spot 1 indicated that it is a dimer of pro-ANF since it possesses the same immunoreactivity, the same charge, double its mass, and can be converted with dithiothreitol into a 16.8-kDa pro-ANF form. Alkaline phosphatase and protein kinase A treatments indicated that spots 5, 6, and 7 are probably not phosphorylated forms of pro-ANF. Carboxypeptide A and B treatments in conjunction with amino acid analysis suggested that spot 7 is ANF-(1-128); spot 6, the major one, ANF-(1-126); and spot 5, ANF-(1-123) or ANF-(1-124). Water deprivation or morphine injection, two maneuvers which are known to influence ANF secretion and atrial ANF content, failed to affect the molecular heterogeneity of pro-ANF except for spot 1. The formation of the dimer appeared to be time-dependent. These results emphasize the heterogeneity of the pro-ANF molecule stored in atrial granules. We suggest that this heterogeneity may be due, in part, to the action of some proteases, such as carboxypeptidase E or a tripeptidyl carboxyhydrolase.     

Specialist deterrent chemoreceptors enable Pieris caterpillars to discriminate between chemically different deterrents

Pieris butterflies (Lepidoptera: Pieridae) are specialist herbivores of cruciferous plants. They exploit glucosinolates, secondary plant metabolites chemotaxonomically characteristic for this plant family, as token stimuli. In addition to particular glucosinolates, some genera of the Cruciferae contain cardenolides, steroidal allelochemicals that act as potent feeding and oviposition deterrents to several Pieris species. We investigated the sensory mechanisms by which these compounds are perceived in larvae. Pieris caterpillars and many other lepidopterous species are endowed with so-called generalist deterrent receptors, that respond to a broad spectrum of secondary plant substances. In Pieris caterpillars we found a second type of deterrent chemoreceptor in maxillary styloconic taste sensilla. This neuron is very sensitive to cardenolides (threshold 0.1–0.3 M). The generalist deterrent receptor also responds to these substances but its threshold lies at 50–100× higher concentrations. In behavioural preference experiments Pieris brassicae L. caterpillars preferred cardenolide-treated cabbage leaf discs when confronted with a choice between them and a deterrent substance that does not occur in the Brassicaceae. The cardenolides acted as potent deterrents when offered against untreated cabbage leaf discs. This demonstrates that the balance of activity elicited in the two types of deterrent chemoreceptors determines the behavioural decision.     

Evolution and losses of spines in slug caterpillars (Lepidoptera: Limacodidae)

Larvae of the cosmopolitan family Limacodidae, commonly known as “slug” caterpillars, are well known because of the widespread occurrence of spines with urticating properties, a morpho‐chemical adaptive trait that has been demonstrated to protect the larvae from natural enemies. However, while most species are armed with rows of spines (“nettle” caterpillars), slug caterpillars are morphologically diverse with some species lacking spines and thus are nonstinging. It has been demonstrated that the evolution of spines in slug caterpillars may have a single origin and that this trait is possibly derived from nonstinging slug caterpillars, but these conclusions were based on limited sampling of mainly New World taxa; thus, the evolution of spines and other traits within the family remains unresolved. Here, we analyze morphological variation in slug caterpillars within an evolutionary framework to determine character evolution of spines with samples from Asia, Australia, North America, and South America. The phylogeny of the Limacodidae was reconstructed based on a multigene dataset comprising five molecular markers (5.6 Kbp: COI, 28S, 18S, EF‐1α, and wingless) representing 45 species from 40 genera and eight outgroups. Based on this phylogeny, we infer that limacodids evolved from a common ancestor in which the larval type possessed spines, and then slug caterpillars without spines evolved independently multiple times in different continents. While larvae with spines are well adapted to avoiding generalist predators, our results imply that larvae without spines may be suited to different ecological niches. Systematic relationships of our dataset indicate six major lineages, several of which have not previously been identified.