Camouflage and selective predation in caterpillars of the poplar and eyed hawkmoths (Laothoe populi and Smerinthus ocellata)

Final instar caterpillars of the poplar hawkmoth ( Laothoe populi ) are usually yellow-green when they have been feeding on Salix fragilis but white when on Populus alba. Similarly final instar caterpillars of the eyed hawkmoth (Smerinthus ocellata) vary in colour from yellow-green when fed on Salix cinerea or S. fragilis to grey-green when fed on Salix ripens or S. alba. Most caterpillars of these two moths are thus very cryptic to the human eye. It has already been shown that colour matching is brought about by young caterpillars changing colour depending on the colour of the substrate perceived by their eyes. Evidence is presented here for selective predation, probably by birds. Yellow-green poplar hawk caterpillars disappeared from P. alba more rapidly than did white caterpillars, but there was no significant difference in the survival of the two colour forms on S. fragilis. Yellow-green caterpillars survived better on S. fragilis than on P. alba , but there was no significant difference in the survival of white caterpillars on the two plants. Reasons why selective predation did not occur in all of the experiments are discussed.     

Garotenoids and colouration of poplar hawkmoth caterpillars (Laothoe populi)

Carotcnoids and chlorophylls a and b were extracted from final instar caterpillars of the poplar hawkmoth ( Laothoe populi ) and the eyed hawkmoth ( Smerinthus ocellata ), as well as from their food plants. Both species of caterpillar absorb the two chlorophylls and the carotenoids lutein, cis -lutein and β-carotene in the gut and deposit lutein and cis -lutein in the integument. It is the lutein, together with pterobilin, that is largely responsible for the colour of the insect: yellow-green poplar hawkmoth caterpillars have more lutein in the integument than dull green ones which in turn have more than white ones. Yellow-green and dull green caterpillars both sequester lutein and cis -lutein in the gut wall, but the yellow-greens translocate more of these pigments to the integument than the dull greens. The white caterpillars absorb very little lutein and cis -lutein into the gut, and so they have much less also in the integument. The mechanism by which the reflected light perceived by the caterpillar is translated into differential absorption of pigment by the gut and deposition in the integument is not known.     

Foodplant effects on colour morphs of Eumorpha fasciata caterpillars (Lepidoptera: Sphingidae)

Caterpillars of the hawkmoth Eumorpha fasciata are highly polymorphic for colour, with green, pink, and pink-and-yellow forms in the second through fourth instars, and green and multicoloured forms in the fifth instar. Four years of field censuses on four foodplant species determined that all morphs were found on all plant species; morph frequencies were homogeneous on each plant species over time; and morph frequencies differed consistently among plant species. When larvae were reared from eggs on three of the hostplant species in the laboratory, differences in morph frequencies paralleled the census results. Thus foodplant quality is one factor affecting larval colour in E. fasciata. A literature survey reveals that foodplant effects on larval colouration may be widespread in the family Sphingidae, but most reports are anecdotal rather than experimental. The implications of this mechanism of colour determination are discussed.     

The lethal plant defense paradox remains: inducible host-plant aristolochic acids and the growth and defense of the pipevine swallowtail

Toxic plants with sequestering specialists are presented with a problem because plant derived toxins protect herbivores against natural enemies. It has been suggested that early induction of toxins and later relaxation of these defenses may help the plant resolve this problem because neonate caterpillars incur the physiological cost of dealing with toxins in early life, but are denied toxins when they are able to sequester them efficiently. In California, the pipevine swallowtail, Battus philenor L. (Lepidoptera: Papilionidae), feed exclusively on Aristolochia californica Torrey (Aristolochiaceae), an endemic vine that contains toxic alkaloids called aristolochic acids that caterpillars sequester to provide chemical defense in immature and adult stages. In a field experiment, the concentration of aristolochic acids doubled in the plant following leaf damage and returned to constitutive levels after six days. Neonate pipevine swallowtail caterpillars showed no aversion to high levels of aristolochic acid in a preference test. Caterpillars reared on leaves with supplemented aristolochic acid showed no physiological cost or increased mortality compared to caterpillars reared on un-supplemented leaves. Searching efficiency and capture rate of lacewing larvae (Chrysoperla), a common predator of first instar caterpillars, was compromised significantly after feeding on caterpillars reared on leaves with supplemented concentrations of aristolochic acid compared to caterpillars feeding on control plants. Additionally, mortality of lacewings increased when they were provided with a diet of B. philenor caterpillars reared on supplemented leaves compared to caterpillars reared on control leaves. Thus, the induction of aristolochic acids in the plant following leaf damage does not resolve the problem confronted by the plant and may confer benefits to this sequestering specialist.     

Effect of light quality and larval diet on morph induction in the polymorphic caterpillar Nemoria arizonaria (Lepidoptera: Geometridae)

The emerald moth Nemoria arizonaria (Geometridae) is bivoltine, with distinct broods of caterpillars hatching in the spring and summer. Caterpillars of the spring brood develop into mimics of oak catkins, while caterpillars of the summer brood develop into mimics of oak twigs. Previous rearing experiments showed that all caterpillars reared on oak catkins developed into catkin morphs, while all caterpillars reared on oak leaves developed into twig morphs, regardless of temperature or photoperiod. However, those previous rearing experiments did not control the colour of light perceived by the caterpillars independently of their dict. Since wavelengths of light perceived by some species of polymorphic caterpillars can influence their colour, it is possible that morph induction in Nemoria arizonaria is due to the characteristics of light reflected from yellow catkins or green leaves, rather than larval diet itself. The experiments reported here independently varied larval diet and light characteristics to determine if light quality is involved in morph induction. Only larval diet influenced morph induction, since all caterpillars reared on catkins developed into the catkin morph, and all caterpillars reared on oak leaves developed into the twig morph, regardless of whether they perceived yellow light, green light, or were raised in the dark.     

The capacity of a Myrmica ant nest to support a predacious species of Maculinea butterfly

Summary Caterpillars of Maculinea arion are obligate predators of the brood of Myrmica sabuleti ants. In the aboratory, caterpillars eat the largest available ant larvae, although eggs, small larvae and prepupae are also palatable. This is an efficient way to predate. It ensures that newly-adopted caterpillars consume the final part of the first cohort of ant brood in a nest, before this pupates in early autumn and becomes unavailable as prey. At the same time, the fixed number of larvae in the second cohort is left to grow larger before being killed in late autumn and spring. Caterpillars also improve their feeding efficiency by hibernating for longer than ants in spring, losing just 6% of their weight while the biomass of ant larvae increases by 27%. Final instar caterpillars acquire more than 99% of their ultimate biomass in Myrmica nests, growing from 1.3 mg to an estimated 173 mg. A close correlation was found between the weights of caterpillars throughout autumn and the number of large ant larvae they had eaten. This was used to calculate the number of larvae eaten in spring, allowing both for the loss of caterpillar weight during winter and the increase in the size of their prey in spring. It is estimated that 230 of the largest available larvae, and a minimum nest size of 354 M. sabuleti workers, is needed to support one butterfly. Few wild M. sabuleti nests are this large: on one site, it was estimated that 85% of nests were too small to produce a butterfly, and only 5% could support two or more. This prediction was confirmed by the mortalities of 376 caterpillars in 151 wild M. sabuleti nests there. Mortalities were particularly high in nests that adopted more than two caterpillars, apparently due to scramble competition and starvation in autumn. Survival was higher than predicted in wild nests that adopted one caterpillar. These caterpillars seldom exhaust their food before spring, when there is intense competition among Myrmica for nest sites. Ants often desert their nests in the absence of brood, leaving the caterpillar behind. Vacant nests are frequently repopulated by a neighbouring colony, carrying in a fresh supply of brood. Maculinea arion caterpillars have an exceptional ability to withstand starvation, and sometimes survive to parasitize more than one Myrmica colony. Despite these adaptations, predation is an inefficient way to exploit the resources of a Myrmica nest. By contrast, Maculinea rebeli feeds mainly at a lower trophic level, on the regurgitations of worker ants. Published data show that Myrmica nests can support 6 times more caterpillars of Maculinea rebeli than of M. arion in the laboratory. This is confirmed by field data.     

Effects of elevated temperature and carbon dioxide on the nutritional quality of leaves of oak (Quercus robur L.) as food for the Winter Moth (Operophtera brumata L.)

1. Pedunculate Oak trees were grown in ambient and elevated temperatures and CO₂. Leaves were fed to Winter Moth caterpillars reared either in constant conditions or with the trees (caged or on-tree).
2. Caterpillars in constant conditions ate the same mass and produced the same mass of faeces whether fed elevated or ambient temperature leaves. However, less was assimilated from elevated leaves, resulting in lighter pupae and fewer, lighter eggs.
3. Caterpillars in constant conditions ate more and produced more faeces when fed elevated CO₂ leaves than when fed ambient CO₂ leaves, but the mass assimilated and pupal mass were unchanged.
4. Caged caterpillars reared with the trees from which they were fed had constant pupal mass in all treatments, but pupated earlier at elevated temperature. Pupal mass was also unaffected when caterpillars fed on the trees.
5. Nitrogen was reduced in both elevated temperature and elevated CO₂ leaves. Increased fibre in the former prevented increased consumption and resulted in reduced pupal mass and fecundity. Reduced fibre in the latter allowed increased consumption, resulting in pupae of normal mass.
6. Despite the clear effect of nutrient quality, experiments rearing caterpillars and trees together suggest that anticipated climatic change will have no nutritional effect on Winter Moth development.     

Density‐dependent prophylaxis in primary anti‐parasite barriers in the velvetbean caterpillar

1. Organisms rely on a set of primary barriers to prevent invasion by parasites, and secondary defences to fight parasites that breach the primary barriers. However, maintaining these defences to be active and effective is costly. Thus, hosts increase investment in anti‐parasite defences under situations of high risk of infection and reduce defences when the risk is reduced (the ‘Density‐Dependent Prophylaxis’ hypothesis). 2. In the present study, it was tested whether the midgut primary defences of the velvetbean caterpillar Anticarsia gemmatalis Hübner present density‐dependent plasticity, and also whether these defences could be induced by a viral pathogenic challenge. The aim was to examine whether morphometry and the structure of the midgut and peritrophic matrix (PM) change in accordance with colour transition in caterpillars, and whether such changes may provide the caterpillars a more protective barrier against invasion by Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV). 3. It was found that PM and the midgut epithelium of the velvetbean caterpillar change plastically according to phenotype, itself a response to changes in population density. Caterpillars reared at high densities (black phenotype) had a considerably thicker midgut epithelia and peritrophic matrices than those reared individually (green phenotype), and there was also more chitin in the PM of the former. 4. This was interpreted as the first demonstration of increased investment in primary, barrier, defences against parasites, in response to increased conspecific density and an increased risk of infection. The possibility that this arises as a positive result of pleiotropy is discussed further, wherein the biochemical pathways responsible for the up‐regulation of the immune system are also involved in midgut properties.     

The influence of diet on growth and secretion behaviour of myrmecophilous Polyommatus icarus caterpillars (Lepidoptera: Lycaenidae)

Abstract.

• 1 Caterpillars of the facultatively myrmecophilous butterfly Polyommatus icarus were reared on inflorescences, or foliage, of four natural hostplant species and on an artificial diet to study dietary effects on larval growth and secretory capacity.
• 2 Caterpillars achieved highest weights and relative growth rates when fed flowers of Medicago sativa, Lotus corniculatus or Melilotus officinalis. Larvae reared on Coroniüa varia (flowers and leaves), foliage of M.sativa and on the artificial diet pupated at lower weights and achieved lower growth rates.
• 3 In standardized experiments with the ant species Lasius flavus, secretion rates from the dorsal nectar organ (DNO) were 2 times higher among flower-fed caterpillars than among foliage-fed siblings or caterpillars on the artificial diet. Larvae reared on C.varia flowers were superior to all other food treatments with respect to secretion rates.
• 4 High water content of larval diet, as in flowers, appears to be important for lycaenid caterpillars to achieve high secretion rates, whereas the correlation between myrmecophily and nutrient availability, as evidenced by growth rates, was less pronounced.
• 5 Using experimental data on larval growth and secretion rates, the lifetime volume of secretions from the DNO is estimated to range from 2 to 5 μl in most food treatments. Only on C.varia flowers (5.5–8.7 μl) and on M.sativa leaves (0.9–1.1 μl) did the caterpillars deviate in their absolute investment in myrmecophily.
• 6 The estimated lifetime investment accounted for 1.6–5.5% of prepupal fresh weight in all food treatments except on C.varia flowers (7.8–12.3%).

     

A biotrophic fungal infection of the great burnet Sanguisorba officinalis indirectly affects caterpillar performance of the endangered scarce large blue butterfly Phengaris teleius

Interactions between ecological communities of herbivores and microbes are commonly mediated by a shared plant. A tripartite interaction between a pathogenic fungus-host plant-herbivorous insect is an example of such mutual influences. In such a system a fungal pathogen commonly has a negative influence on the morphology and biochemistry of the host plant, with consequences for insect herbivore performance. Here we studied whether the biotrophic fbngus Podosphaera ferruginea, attacking the great burnet Sanguisorba officinalis, affects caterpillar performance of the endangered scarce large blue butterfly Phengaris teleius. Our results showed that the pathogenic ftmgus affected the number and size of inflorescences produced by food-plants and, more importantly, had in direct, plant-mediated effects on the abun dance, body mass and immune response of caterpillars. Specifically, we found the relationship between caterpillar abundance and variability in inflorescence size on a plant to be positive among healthy food-plants, and negative among infected food-plants. Caterpillars that fed on healthy food-plants were smaller than those that fed on infected food-plants in one studied season, while there was no such difference in the other season. We observed the relationship between caterpillar immune response and the proportion of infected great burnets within a habitat patch to be positive when caterpillars fed on healthy food-plants, and negative when caterpillars fed on infected food-plants. Our results suggest that this biotrophic fungal infection of the great burnet may impose a significant indirect influence on P. teleius caterpillar performance with potential consequences for the population dynamics and structure of this endangered butterfly.     

Comparative life history and parasitism of a new colour morph of the walnut aphid in California

1 The walnut aphid Chromaphis juglandicola is a yellow aphid. In 2003, however, a white colour morph was discovered in the Sacramento Valley of California. The colour dimorphism occurs between clone lines and, when white morphs are present, they occur in mixed colour morph colonies on the underside of walnut leaves. 2 Laboratory experiments were undertaken to evaluate the thermal requirements for development, adult longevity and progeny production of the two colour morphs. Host instar preference of Trioxys pallidus, a parasitoid responsible for the successful biological control of the walnut aphid in California, was examined separately for each colour morph, and host colour preference was investigated for the preferred instar. 3 No differences in thermal requirements for development, adult size or mean longevity were detected between yellow and white colour morphs. A small difference in early reproduction was detected: white colour morphs produced more progeny on each of the two first days of adult reproduction than yellow colour morphs. 4 Trioxys pallidus showed a slight preference for the fourth instar of the yellow morph over the second‐ and third‐, but equal preference for second, third and fourth instars of the white morph. When offered equal numbers of fourth instars of the two colour morphs, T. pallidus did not show any colour preference. 5 The differences in early aphid reproduction and host instar preference by T. pallidus were combined in a stage‐structured matrix model. Model analysis showed a greater potential for population growth of the white morph over the yellow morph, with early reproduction having a greater influence than host instar preference.     

Nuclear polyhedrosis virus as a biological control agent for Malacosoma americanum (Lepidoptera: Lasiocampidae)

In addition to damaging trees, the eastern tent caterpillar is implicated in early fetal loss and late‐term abortion in horses. In a field study, we evaluated the potential biological control of the caterpillar using eastern tent caterpillar nuclear polyhedrosis virus (ETNPV), a naturally occurring virus that is nearly species‐specific. Egg masses were hatched and second instar larvae were fed virus‐inoculated foliage to propagate the virus in vivo. Then, a viral pesticide was formulated at concentrations of 10⁴, 10⁶ and 10⁸ polyhedral inclusion bodies per ml. The pesticide was applied to foliage on which second, third and fourth instar caterpillars were feeding. When the majority of surviving larvae reached the sixth instar, colonies were collected and the surviving caterpillars counted. Mean numbers of surviving caterpillars per treatment were compared via 95% bootstrap confidence intervals. The data indicate second instar caterpillars were highly susceptible to the virus, but only at the highest concentration tested. Third instar caterpillars were also somewhat susceptible to high virus concentrations, while fourth instar caterpillars were fairly resistant. Our data provide the strongest evidence to date that ETNPV can be propagated, harvested and refined for formulation as a biological control agent for eastern tent caterpillar. Its use on this insect may be merited in circumstances where landowners and managers need to protect trees and horses.     

Social Cues and Following Behavior in the Forest Tent Caterpillar

Colonies of the social caterpillar Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae) travel in groups following silk trails marked with pher-omone. This study examined first, the cues involved in following behavior and second, the responses to these cues at different larval stadia. Both second and fourth instar larvae discriminated between fresh and older trails, and travelled faster in the presence of trails. In addition to trail following, young caterpillars exhibited leader following, which might be particularly important in exploring unmarked territory. Indeed, second instar caterpillars were more likely to travel together when trails were absent. Fourth instar larvae exhibited greater independent locomotion in the absence of trails than did younger larvae. These findings help explain patterns of social behavior observed in forest tent caterpillar colonies in the field.     

Behavioural ecology of defence in a risky environment: caterpillars versus ants in a Neotropical savanna

1. Predatory ants may reduce infestation by herbivorous insects, and slow‐moving Lepidopteran larvae are often vulnerable on foliage. We investigate whether caterpillars with morphological or behavioural defences have decreased risk of falling prey to ants, and if defence traits mediate host plant use in ant‐rich cerrado savanna. 2. Caterpillars were surveyed in four cerrado localities in southeast Brazil (70–460 km apart). The efficacy of caterpillar defensive traits against predation by two common ant species (Camponotus crassus, C. renggeri) was assessed through experimental trials using caterpillars of different species and captive ant colonies. 3. Although ant presence can reduce caterpillar infestation, the ants' predatory effects depend on caterpillar defence traits. Shelter construction and morphological defences can prevent ant attacks (primary defence), but once exposed or discovered by ants, caterpillars rely on their size and/or behaviour to survive (secondary defence). 4. Defence efficiency depends on ant identity: C. renggeri was more aggressive and lethal to caterpillars than C. crassus. Caterpillars without morphological defences or inside open shelters were found on plants with decreased ant numbers. No unsheltered caterpillar was found on plants with extrafloral nectaries (EFNs). Caterpillars using EFN‐bearing plants lived in closed shelters or presented morphological defences (hairs, spines), and were less frequently attacked by ants during trials. 5. The efficiency of defences against ants is thus crucial for caterpillar survival and determines host plant use by lepidopterans in cerrado. Our study highlights the effect of EFN‐mediated ant‐plant interactions on host plant use by insect herbivores, emphasizing the importance of a tritrophic viewpoint in risky environments.     

黄野螟群体大小对幼虫个体生存和发育的影响

幼虫聚集取食是一些鳞翅目昆虫提高幼虫生长和存活的手段。通过对聚集取食的黄野螟Heortia vitessoides Moore幼虫在室内条件下进行饲养,探究黄野螟幼虫的群体数量大小(分别为30、60、90头幼虫)对幼虫体长、发育速度、达末龄(5龄)时的存活率的影响。分析发现在排除天敌捕食和食物缺乏的前提下,从3龄开始,大群体(90头)的体长显著大于小群体(30头),大群体的幼虫发育速度显著快于小群体,大群体对幼虫前4个龄期时的成活率没有显著提高,尽管大群体的存活率在均值上高于小群体,但是幼虫总体存活率在不同群体中差异不显著。黄野螟幼虫的聚集取食对幼虫的生长的促进作用明显,显著促进了幼虫的生长,加快了幼虫的发育。     

Temperature and food quality effects on growth,consumption and post-ingestive utilization efficiencies of the forest tent caterpillar Malacosoma disstria (Lepidoptera: Lasiocampidae)

Temperature and food quality can both influence growth rates, consumption rates, utilization efficiencies and developmental time of herbivorous insects. Gravimetric analyses were conducted during two consecutive years to assess the effects of temperature and food quality on fourth instar larvae of the forest tent caterpillar Malacosoma disstria Hübner. Larvae were reared in the laboratory at three different temperatures (18, 24 and 30 degrees C) and on two types of diet; leaves of sugar maple trees Acer saccharum Marsh. located at the forest edge (sun-exposed leaves) or within the forest interior (shade-exposed leaves). In general, larvae reared at 18 degrees C had lower growth rates and lower consumption rates than larvae reared at the warmer temperatures (24 and 30 degrees C). Moreover, the duration of the instar decreased significantly with increasing temperatures. Type of diet also affected the growth rates and amount of food ingested by larvae but did not affect the duration of the instar. Larvae fed sun-exposed leaves consumed more food and gained higher biomasses. Values of approximate digestibility and efficiency of conversion of ingested food were also higher when larvae were fed sun-exposed leaves. Higher growth rates with increasing temperatures were primarily the result of the shorter stadium duration. The higher growth rates of larvae fed sun-exposed leaves were possibly the result of stimulatory feeding and consequently greater food intake and also a more efficient use of food ingested. This study suggests that the performance of M. disstria caterpillars could be enhanced by warmer temperatures and higher leaf quality.     

Leaf age effects of elevated CO2-grown white oak leaves on spring-feeding lepidopterans

Folivorous insect responses to elevated CO₂-grown tree species may be complicated by phytochemical changes as leaves age. For example, young expanding leaves in tree species may be less affected by enriched CO₂-alterations in leaf phytochemistry than older mature leaves due to shorter exposure times to elevated CO₂ atmospheres. This, in turn, could result in different effects on early vs. late instar larvae of herbivorous insects. To address this, seedlings of white oak (Quercus alba L.), grown in open-top chambers under ambient and elevated CO₂, were fed to two important early spring feeding herbivores; gypsy moth (Lymantria dispar L.), and forest tent caterpillar (Malacosoma disstria Hübner). Young, expanding leaves were presented to early instar larvae, and older fully expanded or mature leaves to late instar larvae. Young leaves had significantly lower leaf nitrogen content and significantly higher total nonstructural carbohydrate:nitrogen ratio as plant CO₂ concentration rose, while nonstructural carbohydrates and total carbon-based phenolics were unaffected by plant CO₂ treatment. These phytochemical changes contributed to a significant reduction in the growth rate of early instar gypsy moth larvae, while growth rates of forest tent caterpillar were unaffected. The differences in insect responses were attributed to an increase in the nitrogen utilization efficiency (NUE) of early instar forest tent caterpillar larvae feeding on elevated CO₂-grown leaves, while early instar gypsy moth larval NUE remained unchanged among the treatments. Later instar larvae of both insect species experienced larger reductions in foliage quality on elevated CO₂-grown leaves than earlier instars, as the carbohydrate:nitrogen ratio of leaves substantially increased. Despite this, neither insect species exhibited changes in growth or consumption rates between CO₂ treatments in the later instar. An increase in NUE was apparently responsible for offsetting reduced foliar nitrogen for the late instar larvae of both species.     

Leaf toughness changes the effectiveness of larval aggregation in the butterfly Byasa alcinous bradanus (Lepidoptera: Papilionidae)

Two subspecies of the papilionid butterfly Byasa alcinous , B. a. bradanus and B. a. alcinous , have varying degrees of larval aggregation. Early instar larvae of ssp. bradanus always occur in aggregations. To determine the functions of larval aggregation in this subspecies, we examined the effects of leaf toughness on larval performance when caterpillars were reared alone and in aggregations. Newly hatched larvae were reared either individually or in groups of 10 and were fed either tough or tender leaves of Aristolochia debilis . When fed tough leaves, more gregarious larvae survived the first instar. This difference between solitary and aggregated larvae did not occur when caterpillars were fed soft leaves. The effects of aggregation on larval weight and duration were not significant between leaf-toughness treatments. Larval aggregation of B. a. bradanus improves larval survivorship in early instars that use host plants with tough leaves.     

Effects of ambient colour on colour preference and growth of juvenile rainbow trout Oncorhynchus mykiss (Walbaum)

Colour preference of individual juvenile rainbow trout Oncorhynchus mykiss was tested at 1 and 12° C, and also at 12° C after a 42 day growth experiment under white, blue, green, yellow or red ambient colour. All experiments were carried out under controlled laboratory conditions and the preference was assessed by the location of the fish in a preference tank with four chambers. Rainbow trout showed a preference for blue and green at 1° C and for green at 12° C. After the growth experiment the fish reared in blue tanks preferred blue and green but green was the most preferred colour for the fish reared in green, yellow and red tanks. Yellow and especially red chambers were avoided, irrespective of the ambient colour during the growth trial. The final mass of fish reared in the red aquaria was significantly smaller than that of the fish in green tanks. In addition, when the data of the preference tests were correlated with the data of the growth experiment using mean values of the four tested colours, a very good linear relationship was observed between the preference ( i.e. visit frequency in coloured compartments) and growth rate as well as food intake. When considering the results both from the preference and growth trials it is suggested that green is the best environmental colour for rearing juvenile rainbow trout while rearing in a red environment cannot be recommended.     

Analysis of leaf-rolling behavior ofCaloptilia serotinella (Lepidoptera: Gracillariidae)

The caterpillarCaloptilia serotinella generates the force required to roll leaves by stretching the silk strands it fixes between opposable plant surfaces. The Young's modulus of strands drawn by caterpillars at an average rate of 16 mm s^–1 was 1.1×10⁸ N m^–2. Single strands stretched in a tensiometer had a final Young's modulus of 1.4×10⁹ N m^–2 and withstood a maximum force of 60 × 10^–5 N (i.e., a 60-mg force) before breaking at 30% extension. Strands stretched approximately 14% beyond their equilibrium length by rolling caterpillars exerted an average axially retractive force of 3.2×10^–5 N and drew the leaf 7×10^–3 mm into the roll. During episodes of rolling, the caterpillars spun hundreds of strands capable of generating a collective force in excess of 0.1 N. Potential forces associated with wet contraction of strands were not harnessed by the caterpillar when rolling but subsequent supercontraction of the strands caused them to bind the roll tightly. Caterpillars appeared to facilitate leaf rolling by weakening the midrib with their mandibles.