Effects of methoprene and juvenile hormone on larval ecdysis,emergence, and metamorphosis of the endoparasitic wasp, Apanteles congregatus

Topical application of juvenile hormone I and III or the hormone analogue methoprene to parasitized Manduca sexta larvae inhibited subsequent emergence of the endoparasitic wasp Apanteles congregatus. Methoprene treatment inhibited wasp emergence in a dose-dependent manner, causing either a delay or total inhibition of emergence. These results were interpreted as reflecting inhibitory effects of juvenile hormone on the second-larval ecdysis of the parasitoid that normally occurs during emergence from the host larva. Parasitoid ecdysis was disrupted even when methoprene was applied to host larvae a few hours prior to the normal expected time of emergence. A correlation between the number of emerging parasitoids and the timing of emergence was seen in methoprene-treated hosts, and few parasitoids emerged after day 9 of the host's fifth-instar. Our findings suggest that the suppression of emergence by juvenile hormone analogues noted in previous studies may be due to a similar inhibitory effect on parasitoid ecdysis. We also observed that parasitoids emerging from hosts treated with a low dose of methoprene (1 μg) later pupated normally but then formed nonviable pupal-adult intermediates. Thus use of this insect growth regulator must be undertaken carefully to prevent possible adverse effects on natural parasitoid populations.     

Influence of calcium on Manduca sexta plasmatocyte spreading and network formation

Plasmatocytes are a class of insect hemocytes important in the cellular defense response. In some species, they are phagocytic, protecting the insect from smaller pathogens. In many insects, they work in concert with other hemocytes (particularly other plasmatocytes and granular cells) to form nodules and to encapsulate foreign material. To perform these functions, plasmatocytes attach to, spread on, and surround suitable targets. Because of their importance, because we had previously observed that prolonged incubation of hemocytes in solutions containing the divalent cation chelator ethylenediaminetetraacetic acid (EDTA) inhibited plasmatocyte spreading, and because of the importance of divalent cations in many immune-related functions, we investigated the effect of calcium and magnesium on spreading of plasmatocytes from fifth instar Manduca sexta larvae. On glass slides, plasmatocytes spread more quickly and elongated in Grace's medium containing 5 mM calcium, compared to calcium-free medium. In the presence of calcium, plasmatocyte adhesion, spreading, and network formation were not visibly different in magnesium-free and magnesium-containing Grace's medium. Using immunomicroscopy with a monoclonal antibody specific for plasmatocytes, we measured the length and width of plasmatocytes incubated with several different concentrations of calcium. Plasmatocyte length positively correlated with calcium concentration to 5 mM (maximum concentration tested and approximately the hemolymph concentration). Mean plasmatocyte width was less in 0 and 5 mM calcium than in 0.05 or 0.5 mM calcium. On plastic, hemocytes survived longer than on glass (they survived beyond 24 h) and, in 5 mM calcium, formed an extensive network readily visible by phase-contrast microscopy. This network was never as extensive in the absence of calcium. Network formation in the absence of magnesium, but presence of calcium, resembled network formation in standard Grace's medium.     

A comparative study of the relationship between host size and brood size in Apanteles spp. (Hymenoptera: Braconidae)

ABSTRACT. 1. Current models of insect oviposition predict that clutch size in parasitoids should correlate with host size, with a continuum from solitary species at one end to large gregarious broods at the other. This prediction is tested for the genus Apanteles (sensu lato).
2. The distribution of brood sizes in Apanteles is bimodal, with peaks at one (solitary species) and at about twenty (gregarious species).
3. Brood size of gregarious species correlates with host size, but when a measure of the total volume of a parasitoid brood is plotted against host size, solitary species do not lie on the same regression slope as gregarious species.
4. There is a relative shortage of gregarious species on small hosts, and a relative excess of solitary species on large hosts. Solitary species on large hosts do not fully consume the host resource.
5. The possible role of evolutionary constraints to adaptive progeny allocation in Apanteles is discussed.     

Effects of dietary nicotine on the development of an insect herbivore, its parasitoid and secondary hyperparasitoid over four trophic levels

Abstract.  1. Allelochemicals in herbivore diet are known to affect the development of higher trophic levels, such as parasitoids and predators.
2. This study examines how differing levels of nicotine affects the development of a herbivore, its parasitoid and secondary hyperparasitoid over four trophic levels. Separate cohorts of the herbivore, Manduca sexta , were fed on artificial diets containing 0.0, 0.1, and 0.5% wet weights of nicotine. Some of the larvae in each cohort were separately parasitised in the first (L1) and third (L3) instars by the gregarious endoparasitoid, Cotesia congregata . Newly emerged parasitoid cocoons were, in turn, parasitised by the hyperparasitoid, Lysibia nana .
3. Pupal mass in M . sexta was negatively correlated with nicotine concentrations in the artificial diet, although larval development time was unaffected.
4. Hyperparasitoid survival was highest when there were low levels of nicotine in the diet of M . sexta . Cocoon mass in C . congregata and adult mass in L . nana were mostly affected by nicotine levels in host diet when L1 M. sexta larvae were parasitised. The effects were slightly stronger on L . nana than on C . congregata , indicating the presence of both qualitative and quantitative effects of nicotine concentration on both species.
5. The results suggest that allelochemicals in herbivore diet can have both direct and indirect effects on the performance of higher trophic levels. However, in multitrophic interactions these effects can vary with the stage of the herbivore attacked by the primary parasitoid, as well as with the strategy employed by the herbivore to deal with plant phytotoxins.     

Effects of light availability on host plant chemistry and the consequences for behavior and growth of an insect herbivore

We examined how light availability influenced the defensive chemistry of tomato (Lycopersicon esculentum: Solanaceae). Tomato plants were grown either in full sunlight or under shade cloth rated at 73%. Leaves from plants grown in full sunlight were tougher, had higher concentrations of allelochemicals (chlorogenic acid, rutin and tomatine), and had less protein than leaves from plants grown in shade. We determined how these differences in host plant quality due to light availability affected the behavior and growth of a Solanaceae specialist, Manduca sexta. Both in the greenhouse and in the field, caterpillars on shade-grown plants grew heavier in a shorter amount of time than those on plants that had previously been grown in full sunlight. In contrast, the effects of previous light availability to plants on caterpillar behavior appeared to be minor.To further investigate how light availability to plants influenced herbivore growth, we examined the effects of leaf-powder diets made from tomato leaves of different ages (new, intermediate, or mature) grown in full sunlight or shade on caterpillar performance. Caterpillars fed diets made from plants grown in shade consumed less but grew faster than larvae fed diets made from tomato plants grown in full sunlight. Caterpillars fed diets made from new leaves grew larger in less time than caterpillars fed diets made from intermediate aged leaves. Caterpillars did not survive on the mature leaf powder diets. There were plant-light treatment by larval thermal regime interactions. For example, at 26:15 °C , plant-light treatment had no effect on stadium duration, but at 21:10 °C, stadium duration was prolonged with the full sunlight-new leaf diet compared with the shaded-new leaf diet. In a second diet experiment, we examined the interactive effects of protein and some tomato allelochemicals (rutin, chlorogenic acid and tomatine) on the performance of caterpillars. There were food quality by thermal regime interactions. For instance, at 26:15 °C , neither protein nor allelochemical concentration influenced stadium duration, whereas at 21:10 °C, stadium duration was prolonged with the low protein-high allelochemical diet, which simulated full sunlight leaves. In sum, light availability to plants affected defensive chemistry and protein concentration. The difference in food quality was great enough to influence the growth of a specialist insect herbivore, but the effects were temperature-dependent.     

Relationship between host diversity and parasite diversity: flea assemblages on small mammals

Aim We examined the relationship between host species richness and parasite species richness using simultaneously collected data on small mammals (Insectivora, Rodentia and Lagomorpha) and their flea parasites. Location The study used previously published data on small mammals and their fleas from 37 different regions. All the world's main geographical regions other than Australasia and Wallacea were represented in the study, i.e. neotropical, nearctic, palaearctic, oriental and afrotropical realms. Methods We controlled the data for the area sampled and sampling effort and then tested this relationship using both cross‐region conventional analysis and the independent contrasts method (to control for the effects of biogeographic historical relationships among different regions). Brooks parsimony analysis was used to construct a region cladogram based on the presence/absence of a host species and host phylogeny. Results Both cross‐region and independent contrasts analyses showed a positive correlation between host species richness and flea species richness. Conventional cross‐region regression under‐ or overestimated fleas species richness in the majority of regions. Main conclusions When the regression derived by the independent contrasts method was mapped onto the original tip data space, points that deviated significantly from the regression originated from Kenya, Mississippi and southern California (lower than expected flea richness) and Chile, Idaho, south‐western California and Kyrgyzstan (higher than expected flea richness). These deviations can be explained by the environmental mediation of host–flea relationships and by a degree of environmental variety in sampled areas.     

Induction of supernumerary larval moulting in the tobacco hornworm Manduca sexta: interaction of bisacylhydrazine ecdysteroid agonists with endogenous Juvenile Hormone

Abstract When given in a critical dietary dose range, the insecticidal bisacylhydrazine ecdysteroid agonists RH‐5849 or tebufenozide (RH‐5992) cause fifth stage Manduca sexta (L.) larvae to moult to a supernumerary sixth‐stage giant larva. The effect is dependent on exposure to the chemicals immediately after the previous ecdysis. Previous removal of the corpora allata does not interfere with the induction of premature moulting by RH‐5849 but completely prevents the formation of supernumerary larvae. The juvenilizing effect is therefore due to the interaction of the moult‐promoting effect of the ecdysteroid agonists with the high titre of endogenous Juvenile Hormone that is present just after ecdysis to the fifth stage in this insect. The ecdysteroid agonists themselves appear to have no intrinsic Juvenile Hormone‐agonist properties. Sixth‐stage larvae resulting from exposure to critical dietary concentrations of RH‐5849 are morphologically completely larval in character. When transferred to diet without the ecdysteroid agonist, they feed normally and gain weight, growing much larger than control fifth stage insects. At the end of the supernumerary stage, they cease to feed, wander in the usual way, and form a normal pupal cuticle but then die as pharate pupae without shedding the sixth‐stage larval cuticle.     

Effect of Maruca vitrata (Lepidoptera: Crambidae) host plants on life‐history parameters of the parasitoid Apanteles taragamae (Hymenoptera: Braconidae)

Abstract The effect of four host plant species of the herbivore Maruca vitrata Fabricius (Lepidoptera: Crambidae) on development time, longevity, fecundity and sex ratio of the parasitoid Apanteles taragamae Viereck (Hymenoptera: Braconidae) was investigated under laboratory conditions. The larvae were parasitized when in the second instar. Maruca vitrata larvae were fed with flowers of four legumes, that is, Vigna unguiculata (cowpea), Sesbania rostrata, Lonchocarpus sericeus and Pterocarpus santalinoides, or an artificial diet both before and after parasitization. The parasitoid did not develop in hosts feeding on L. sericeus or V. unguiculata at 25°C, or on P. santalinoides at 25°C or 29°C. Apanteles taragamae had the shortest development time on artificial diet at both 25°C and 29°C while the longest development time was recorded on L. sericeus at 29°C. Female wasps took longer to develop compared to males at the two temperatures, regardless of the feeding substrate of their host. The longevity of the wasps at 25°C varied among feeding substrates, but not at 29°C. Survival rate of parasitized larvae depends on the feeding substrate. Moreover, infection of host larvae with Maruca vitrata multi‐nucleopolyhedrovirus (MaviMNPV) killed larger proportions of parasitized larvae at 25°C than at 29°C, which was likely caused by the difference in parasitoid developmental rate. The proportion of female parasitoids was lowest on L. sericeus. The daily fecundity showed a nonlinear trend regardless of the feeding substrate, indicating that A. taragamae is a pro‐ovigenic species. The data support the slow growth–high mortality hypothesis.     

菜蛾绒茧蜂和小菜蛾对杀虫剂的敏感性及酶学特性的比较研究

分别采用药膜法和浸叶法测定了菜蛾绒茧蜂Apanteles plutellae和小菜蛾Plutella xylostella对杀虫剂的敏感度。结果显示： 有机磷、氨基甲酸酯、拟除虫菊酯类杀虫剂、阿维菌素和锐劲特对菜蛾绒茧蜂高毒,而抑太保和Bt为低毒,然而,短时间(1 h)接触常规防治剂量的锐劲特、氰戊菊酯、氯氰菊酯和乙酰甲胺磷对菜蛾绒茧蜂低毒。增效剂胡椒基丁醚（PB）、磷酸三苯酯（TPP）和马来酸二乙酯（DEM）对菜蛾绒茧蜂的甲胺磷、克百威、氰戊菊酯、氯氰菊酯、阿维菌素和锐劲特敏感性增效显著,但对抑太保无增效作用。PB的增效作用显著高于TPP 和DEM。PB和TPP对菜蛾绒茧蜂羧酸酯酶(CarE),以及DEM对谷胱甘肽S转移酶(GST)具显著的活体抑制作用,但PB,TPP和 DEM对菜蛾绒茧蜂乙酰胆碱酯酶（AChE）无抑制作用。菜蛾绒茧蜂AChE的米氏常数(K_m)、最大反应速度(V_max)、CarE和GST活性分别为小菜蛾的0.22、2.08、4.60和0.45倍,甲胺磷、敌敌畏和克百威对菜蛾绒茧蜂AChE的双分子速度常数(Ki)分别为对小菜蛾的14.7、10.5 和26.0倍。酶与抑制剂反应温度增高将导致酶抑制率增高,尤其对菜蛾绒茧蜂AChE的抑制作用更为显著。上述结果表明,菜蛾绒茧蜂对有机磷和氨基甲酸酯类杀虫剂的高敏感性与其显著高的AChE敏感性有关,氧化代谢的解毒作用对菜蛾绒茧蜂耐药性的影响大于水解作用。此外,对小菜蛾和菜蛾绒茧蜂杀虫剂敏感性差异的毒理学原因进行了讨论。     

Differentiation of roles of chemosensory organs in food discrimination among host and non-host plants by larvae of the tobacco hornworm, Manduca sexta

ABSTRACT. The contributions of olfactory and gustatory organs in food plant discrimination were examined in larvae of Manduca sexta (Johan.) (Lepidoptera, Sphingidae). Larvae, from which various chemosensory organs had been removed surgically, were tested for feeding preferences for a host, tomato ( Lycopersicon esculentum ); a weakly acceptable non-host, rape ( Brassica napus ); and an unacceptable non-host canna ( Canna generalis ), using a two-choice disc bioassay.
Removal of all known chemosensory organs resulted in failure to show discriminatory behaviour in a strictly chemosensory bioassay, indicating that all external chemosensory organs have been accounted for. The involvement of non-chemosensory organs results in residual discrimination for leaves by individuals with total chemosensory ablations.
Larvae possessing either olfactory or gustatory organs still exhibit normal preferences for tomato over rape. Gustatory (but not olfactory) organs are required for larvae to show normal preferences for tomato over canna; in fact, olfactory organs do not appear to participate in this decision.
To examine which if any of the plant species is being selected in two-choice tests, larvae were given a choice between each leaf species and a 'neutral' substance (wet filter paper). Both olfactory and gustatory organs are required for normal preferences for tomato, but either alone will suffice for rape. Only gustation is needed to select canna, and participation of either the epipharyngeal sensilla or a single medial sensillum styloconicum is sufficient to elicit complete rejection behaviour.
We conclude that, in larvae of M. sexta , the complement of chemosensory organs needed for food plant discrimination varies with the plant species sampled. Evidence is presented exposing a potential artefact of ablation experiments; extirpation of one sensory organ may affect the functioning of others nearby, even though they may appear normal by visual inspection.     

How a haemosporidian parasite of bats gets around: the genetic structure of a parasite,vector and host compared

Parasite population structure is often thought to be largely shaped by that of its host. In the case of a parasite with a complex life cycle, two host species, each with their own patterns of demography and migration, spread the parasite. However, the population structure of the parasite is predicted to resemble only that of the most vagile host species. In this study, we tested this prediction in the context of a vector‐transmitted parasite. We sampled the haemosporidian parasite Polychromophilus melanipherus across its European range, together with its bat fly vector Nycteribia schmidlii and its host, the bent‐winged bat Miniopterus schreibersii. Based on microsatellite analyses, the wingless vector, and not the bat host, was identified as the least structured population and should therefore be considered the most vagile host. Genetic distance matrices were compared for all three species based on a mitochondrial DNA fragment. Both host and vector populations followed an isolation‐by‐distance pattern across the Mediterranean, but not the parasite. Mantel tests found no correlation between the parasite and either the host or vector populations. We therefore found no support for our hypothesis; the parasite population structure matched neither vector nor host. Instead, we propose a model where the parasite's gene flow is represented by the added effects of host and vector dispersal patterns.     

Superinfection and the coevolution of parasite virulence and host recovery

Parasite strategies of host exploitation may be affected by host defence strategies and multiple infections. In particular, within‐host competition between multiple parasite strains has been shown to select for higher virulence. However, little is known on how multiple infections could affect the coevolution between host recovery and parasite virulence. Here, we extend a coevolutionary model introduced by van Baalen (Proc. R. Soc. B, 265, 1998, 317) to account for superinfection. When the susceptibility to superinfection is low, we recover van Baalen's results and show that there are two potential evolutionary endpoints: one with avirulent parasites and poorly defended hosts, and another one with high virulence and high recovery. However, when the susceptibility to superinfection is above a threshold, the only possible evolutionary outcome is one with high virulence and high investment into defence. We also show that within‐host competition may select for lower host recovery, as a consequence of selection for more virulent strains. We discuss how different parasite and host strategies (superinfection facilitation, competitive exclusion) as well as demographic and environmental parameters, such as host fecundity or various costs of defence, may affect the interplay between multiple infections and host–parasite coevolution. Our model shows the interplay between coevolutionary dynamics and multiple infections may be affected by crucial mechanistic or ecological details.     

Comparative morphology of Van der Vecht's organ in Polistes social parasites: host ecology and adaptation of the parasite

Camouflage strategies are common in insect social parasites. Being accepted into an alien colony as a dominant nestmate favours behavioural and morphological adaptations to mimic a specific odour. In Polistes social parasites, abdominal tegumental glands are involved in this camouflage strategy. These glands secreting cuticular hydrocarbons are connected with a modified cuticular area of the last gastral sternite of female wasps, named Van der Vecht's organ, whose secretion is involved in rank and dominance recognition. The size of this exocrine area has been demonstrated to be under selective pressure in Polistes, as a response to an efficient dominance recognition. Because chemical and behavioural integration differs between parasitic species, we carried out a comparison of Van der Vecht's organ size between the three Polistes social parasites and their respective hosts. The parasites Polistes sulcifer and Polistes semenowi, capable of a rapid chemical mimicry and specialized to exploit a lowland host, also show an enlarged Van der Vecht's organ. Conversely, the parasite Polistes atrimandibularis, specialized on a mountain species and showing a slow chemical integration, has a smaller organ. The time available for the parasite to tune up its chemical mimicry, before the emergence of workers to be accepted as a dominant nestmate, appears to be the most important selective pressure acting on the size of this abdominal organ. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013 , 109 , 313–319.     

Studies on binding and uptake of vitellogenin by follicles of the tobacco hornworm,Manduca sexta

An in vitro system for the uptake of ¹²⁵l-vitellogenin (VG) or vitellin into isolated follicles of the tobacco hornworm, Manduca sexta, is described. After incubation with ¹²⁵l-VG, follicles were disrupted and the internal yolk contents separated from the follicle membranes. The results showed that ¹²⁵l-VG was associated principally with the membranes (92%) after incubation at 4°C. However, at 27°C, ¹²⁵l-VG was mainly in the yolk (92%). Furthermore, trypsin treatment removed approximately 70% of VG bound to the follicles at 4°C. Labeled VG was shown to bind to sonicated follicle membranes with high specificity and affinity (K_D ? 1.3 × 10^?8 M). This binding was sensitive to pH and calcium concentration. The total binding sites were estimated at 4 × 10¹⁴ sites/g of membrane protein. Competition studies showed that binding of ¹²⁵l-VG to follicle membranes was blocked by excess unlabeled vitellin and deglycosylated vitellogenin but not by lipophorin (the major hemolymph lipoprotein), microvitellogenin, a female-specific protein (M_r ～ 31,000) found in both hemolymph and eggs, and the smaller vitellogenin subunit, apovitellogenin-II (M_r ～ 45,000). These results suggest that selective uptake of M. sexta VG from the hemolymph involves binding to specific receptors located on the follicle membranes.     

Parasite and host assemblages: embracing the reality will improve our knowledge of parasite transmission and virulence

Interactions involving several parasite species (multi-parasitized hosts) or several host species (multi-host parasites) are the rule in nature. Only a few studies have investigated these realistic, but complex, situations from an evolutionary perspective. Consequently, their impact on the evolution of parasite virulence and transmission remains poorly understood. The mechanisms by which multiple infections may influence virulence and transmission include the dynamics of intrahost competition, mediation by the host immune system and an increase in parasite genetic recombination. Theoretical investigations have yet to be conducted to determine which of these mechanisms are likely to be key factors in the evolution of virulence and transmission. In contrast, the relationship between multi-host parasites and parasite virulence and transmission has seen some theoretical investigation. The key factors in these models are the trade-off between virulence across different host species, variation in host species quality and patterns of transmission. The empirical studies on multi-host parasites suggest that interspecies transmission plays a central role in the evolution of virulence, but as yet no complete picture of the phenomena involved is available. Ultimately, determining how complex host–parasite interactions impact the evolution of host–parasite relationships will require the development of cross-disciplinary studies linking the ecology of quantitative networks with the evolution of virulence.     

Growth and development of Encarsia formosa (Hymenoptera: Aphelinidae) in the greenhouse whitefly, Trialeurodes vaporariorum (Homoptera: Aleyrodidae): effect of host age

The tiny parasitoid wasp, Encarsia formosa, has been used successfully to control greenhouse whiteflies (GHWFs) in greenhouses in many countries throughout the world. Therefore, there has been considerable interest in developing methods for artificially rearing this wasp. However, little information is available concerning the regulation of its development including the host-parasitoid interactions that are required for the parasitoid to complete its life cycle. Here we confirm that parasitoid developmental rates differ significantly based upon the host instar parasitized. Development was faster when 3rd and 4th instar GHWFs were offered for parasitization than when 1st or 2nd instars were used. Our results show that it is primarily the embryo and the first two parasitoid instars that exhibit prolonged developmental times when 1st and 2nd instar whiteflies are parasitized. Although percent emergence was not affected by host age at the time of parasitization, adult longevity as well as adult emergence pattern varied greatly depending upon the instar parasitized. When 3rd and 4th instar GHWFs were selected for oviposition, adult wasps lived significantly longer than when 1st or 2nd instars were used; also, there was a sharp emergence peak on the 2nd day after emergence was first observed (reduced or absent when 1st or 2nd instar GHWFs were parasitized) and the emergence period was reduced from between 8 and 11 days to 5 days. In general, the younger the host instar parasitized, the less synchronous was parasitoid development. Previous reports that E. formosa will not molt to the 2nd instar until the host has reached its 4th instar were not confirmed. When 1st instar host nymphs were parasitized, 2nd instar parasitoids were detected in 3rd instar hosts. Importantly, however, no matter which instar was parasitized, the parasitoid never molted to its last instar until the host had reached Stage 5 of its last instar, a stage in which host pharate adult formation has been initiated. It appears, then, that a condition(s) associated with host pharate adult formation is required for the parasitoid's final larval molt. Results reported here should facilitate the development of in vitro rearing systems for E. formosa.     

Effects of potassium levels over the range occurring in host plants on the growth of an insect herbivore (Manduca sexta)

Third instar tobacco hornworms (Manduca sexta L.: Sphingidae) on low dietary potassium had a lower relative growth rate than individuals on diets with potassium concentrations reflecting those in host-plants, due to decreased consumption rate, lower efficiencies of conversion of ingested and digested food (ECI and ECD), and a prolonged growth/feeding phase. Furthermore, these larvae, when placed on a diet with a moderate potassium concentration through the fourth stadium, ended up being smaller due to lower ECI and less biomass gained, and had a prolonged growth phase, which suggest an irreversible cost of the previous low potassium diet. Third instar hornworms on high potassium diets had lower ECI and ECD, and they had a prolonged growth phase. These individuals, when placed on a moderate potassium diet in the fourth stadium, gained less biomass, than those previously offered hostplant-like-potassium diets. Body potassium concentrations (% dw) at the end of the third stadium were similar among treatment groups. With increasing potassium concentrations in the diet, utilization efficiencies of potassium decreased and potassium concentrations in the frass increased. Correspondingly, water content (% fw) of the newly-molted fourth instar larvae declined with increasing potassium, indicating a passive loss of water during potassium excretion. Low and high dietary potassium reduced survivorship of third instar larvae; fourth instar caterpillars previously fed the low potassium diet also had poor survivorship. We conclude that, within the normal range of potassium concentrations in the hostplants, caterpillar performance is largely unaffected by potassium concentration, but that potassium-poor and potassium-rich diets, such as those hornworms may sometimes experience, can reduce growth and survivorship.     

Ultrastructure of the archigregarine Selenidium vivax (Apicomplexa) - A dynamic parasite of sipunculid worms (host: Phascolosoma agassizii)

Selenidium vivax is a large and unusual unicellular parasite that inhabits the intestinal lumen of the dotted peanut worm, Phascolosoma agassizii. Molecular phylogenies suggest that this archigregarine lineage diverges near the nexus of the apicomplexan radiation and could shed light on to the early evolution of parasitism within the group. The behaviour and ultrastructure of the trophozoites were described using digital videography and scanning and transmission electron microscopy. The trophozoites were extremely flat and capable of dynamic cellular deformations. An intimate association between a superficial layer of mitochondria and longitudinal clusters of subpellicular microtubules formed a distinct functional configuration that helped explain the mechanism behind the cellular motility. Although inconclusive, the presence of small mitochondria-like profiles and narrow connections between larger mitochondrial profiles suggested that an expansive mitochondrial reticulum might surround the trophozoites. The nucleus was highly convoluted and gave rise to blebs of different sizes. The nuclear blebs were connected to the nucleus proper and surrounded by one cisterna of endoplasmic reticulum, giving the impression of four membrane-bound organelles that were misleadingly reminiscent of apicoplasts. The novel attachment apparatus consisted of a transverse ridge, a linear arrangement of pores that contained thread-like structures and a network of dense bodies and endoplasmic reticulum.