浙江丽水中华大蟾蜍和黑眶蟾蜍蝌蚪对水位变化的表型响应

全球气候变暖引发栖息地干涸将对生活在水中的无尾类幼体提出了挑战。通过浙江丽水中华大蟾蜍(Bufo gargarizans)和黑眶蟾蜍(Duttaphrynus melanosticus)蝌蚪在实验条件下对不同水位变化的表型响应,检测表型可塑性的遗传性和环境近因性影响。结果表明,水位变化对中华大蟾蜍蝌蚪早期发育历期、头宽和体重影响不显著,对体长影响显著,其中逐减水位最大、恒低水位最小,慢波、恒高与快波、逐增水位依次减少;水位变化对黑眶蟾蜍蝌蚪早期发育历期、体长、头宽和体重影响均显著;发育历期以恒高水位最大,恒低水位最小;体长以逐减水位最大,恒低、快波和慢波水位显著偏小,逐增和快波水位居中;头宽以恒低水位最小,逐增水位居中,其余较大;体重以恒低水位最小、恒高水位最大,其余居中。水位变化对中华大蟾蜍蝌蚪的变态时间、体长、头宽和体重影响均不显著;水位变化对黑眶蟾蜍蝌蚪的变态时间、体长和体重影响均显著,对头宽影响不显著;恒低水位的变态时间最长,恒高水位的变态时间最短,其他水位变化之间差异不显著;恒高水位的体长最大,恒低和快波水位最小,其他居中;逐增和快波水位的体重最大,恒低水位最小。研究结果表明,繁殖季节不同的中华大蟾蜍和黑眶蟾蜍蝌蚪响应水位变化的表型可塑性差异显著,长期在容易发生干旱和水位变化的冬季繁殖的中华大蟾蜍蝌蚪的表型可塑性低,在雨水充沛的春季繁殖的黑眶蟾蜍蝌蚪的表型可塑性高,表现出表型可塑性的种间差异和遗传性;在早期发育过程中,两种蝌蚪体长的共同的表型变异与缺乏遗传基础的环境近因性影响有关;黑眶蟾蜍蝌蚪对低水位或水位下降作出减速分化的消极响应,响应程度与环境信号的强弱直接相关。     

Basis of selective predation by the aquatic larvae of the salamander, Ambystoma tigrinum

SUMMARY. 1. The effect of several factors on feeding selectivity by three size classes of larvae of Ambystoma tigrinum (Green) was examined. Five types of invertebrate prey were used: Chaoborus, Daphnia , damselflies, Hyalella and water boatmen.
2. Five variables were examined in relation to the electivity indices associated with each prey type: ease of prey consumption, ease of prey capture, relative prey size, prey activity, and microhabitat overlap. No single variable explained the observed electivity.
3. For small larvae (snout-vent length, SVL, <2.5cm) relative prey size was correlated with electivity. For large larvae (SVL >6.5cm), ease of prey consumption, prey activity, and relative prey size predicted electivity values. However, none of the measured variables was related to electivity values found for medium sized larvae (SVL >3.5 and <5.5cm).     

Factors influencing offspring traits in the oviparous multi-clutched lizard,<Emphasis Type="Italic">Calotes versicolor</Emphasis> (Agamidae)

The determinants of offspring size and number in the tropical oviparous multi-clutched lizard,Calotes versicolor, were examined using both univariate and multivariate (path) analyses. InC. versicolor maternal snout-vent length (SVL) and body condition influence clutch mass and clutch size but have no significant influence on offspring size. The positive effect of maternal SVL and body condition on offspring number is counterbalanced by a negative effect of breeding time on egg mass. In fact, breeding time directly influences the offspring body mass and condition through variation in the egg mass. There is a trade-off between offspring mass and condition with offspring number, and breeding time influences both. Offspring hatched from the eggs of early (May–June) or mid (July–August) breeding periods invariably show lower mass and condition than those hatched from the eggs of late breeding season (September–October). Yet, there is no variation in offspring SVL among early, mid and late clutches. Thus, inC. versicolor offspring SVL is optimized while body mass and condition are not optimized.     

Feeding across the food web: The interaction between diet,movement and body size in estuarine crocodiles (Crocodylus porosus)

The estuarine crocodile (Crocodylus porosus) is an apex predator across freshwater, estuarine and coastal environments. The impact of a changing C. porosus population upon the ecosystem is unknown, but due to large ontogenetic changes in body mass (>1000‐fold) their impact may be wide reaching and substantial. Here we investigated the relationship between diet, movement and body size in a population of C. porosus inhabiting a tidal river in northern Australia. Subcutaneous acoustic transmitters and fixed underwater receivers were used to determine the activity space and movement patterns of 42 individuals (202–451 cm in total length). There was no size‐related spatial partitioning among different sized crocodiles. Large individuals (snout–vent length (SVL): 160 cm < SVL < 188.5 cm) did, however, exhibit a much larger activity space than other size classes. Diet and individual specialization was assessed using the composition of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in tissues with different turnover rates. There was a quadratic relationship between body size and δ¹⁵N, suggesting that medium‐sized individuals (110 cm < SVL < 160 cm) incorporated a greater proportion of high trophic prey into their diets than small (SVL < 110 cm) or large individuals (SVL > 160 cm). Tissue δ¹³C composition on the other hand was positively correlated with body size, indicating that different size classes were trophically linked to primary producers in different habitats. Individual‐level analyses showed that small crocodiles were generalist feeders while medium and large size classes specialized on particular prey items within the food webs they fed. The findings further our understanding of ontogenetic variation in C. porosus diet, and suggest that change in C. porosus population size or demographics may be influential at various levels across the local food web.     

Changes in whole body concentrations of thyroid hormones and cortisol in metamorphosing conger eel

Summary To clarify the hormonal regulation of metamorphosis of the conger eel (Conger myriaster), changes in whole body concentrations of thyroid hormones, thyroxine (T₄) and triiodothyronine (T₃), and cortisol during metamorphosis were examined, as well as the changes in the histological activity of the thyroid gland. In larvae before metamorphosis, T₄ and T₃ levels were less than 5 and 0.15 ng·g^-1 respectively. Levels of T₄ increased to about 30 ng·g^-1 during early metamorphosis, and decreased subsequently. Levels of T₃ increased gradually in early metamorphosis, and then increased abruptly to about 2.0 ng·g^-1 in late metamorphosis. Before metamorphosis, cortisol levels of the leptocephali less than 11 cm in total length were greater than 200 ng·g^-1. Cortisol levels decreased rapidly in larger premetamorphic leptocephali, and low levels were maintained throughout the metamorphic period. Histological observation revealed an activation of the thyroid gland in early metamorphosis; thyroid follicle epithelial cells became columnar and their nuclei larger. Active uptake of colloid by these cells and intensive vascularization of the gland were also observed. By the end of metamorphosis, follicle epithelial cells became squamous, indicating a low level of glandular activity. These results suggest that thyroid hormone plays an important role in regulation of conger eel metamorphosis.Abbreviations AL anal length - TL total length - T ₃ triiodothyronine - T ₄ thyroxine     

Are the anticipatory pathways in lecithotrophic larvae that delay metamorphosis adaptations? (A review)

During anticipatory development in lecithotrophic larvae that delay metamorphosis, the growth and differentiation of features of the adult action system continue to develop at a slow pace even though they do not become functional. After metamorphosis occurs, the larger size and advanced development of these components may allow juveniles to initially grow at a faster rate than they normally would. Anticipatory development has been demonstrated in archeogastropods, some solitary ascidians and a hydrozoan. In the gastropod Haliotis and the hydrozoan Phialidium anticipatory development increases the initial growth rate of juveniles. In Haliotis and ascidians all of the larvae of a given female that live long enough exhibit anticipatory development. In Phialidium, the ability of a given female to produce larvae that can exhibit anticipatory development is a maternal polymorphic character. In Haliotis and solitary ascidians that exhibit anticipatory development, it appears to be a slower version of the rapid developmental changes that occur in parts of the adult action system at metamorphosis. In Phialidium, developmental changes in relative sizes of the different presumptive regions of the polyp are slowly altered prior to and independently of metamorphosis. Anticipatory development is not linked to the decrease in the size or nutrient reserves of older larvae but to the length of their larval period. From an evolutionary perspective, the mechanisms that operate during anticipatory development are probably of adaptive significance for lecithotrophic larvae of species that spend variable amounts of time in the water column because of a patchy distribution of appropriate settlement cues. The developmental mechanisms that underlie anticipatory development may have been used during the transition from lecithotrophy to planktotrophy.     

Induction of metamorphosis with potassium ions requires development of competence and an anterior signalling centre in the ascidian Herdmania momus

 Increased K⁺ concentration in seawater induces metamorphosis in the ascidian Herdmania momus. Larvae cultivated at 24°C exhibit highest rates of metamorphosis when treated with 40 mM KCl-elevated seawater at 21°C. At 24°C, H. momus larvae develop competence to respond to KCl-seawater and initiate metamorphosis approximately 3 h after hatching. Larval trunks and tails separated from the anterior papillae region, but maintained in a common tunic at a distance of greater than 60 μm, do not undergo metamorphosis when treated with KCl-seawater; normal muscle degradation does not occur in separated tails while ampullae develop from papillae-containing anterior fragments. Normal programmed degradation of myofibrils occurs when posterior fragments are fused to papillae-containing anterior fragments. These data indicate that H. momus settlement and metamorphosis only occurs when larvae have attained competence, and suggest that an anterior signalling centre is stimulated to release a factor that induces metamorphosis. Received: 15 May 1996 / Accepted: 19 September 1996     

Inhibition of metamorphosis by RFamide neuropeptides in planula larvae of Hydractinia echinata

The primitive nervous system in planula larvae of Hydractinia echinata (Cnidaria) has sensory neurons containing LWamide or RFamide neuropeptides. LWamides have been shown to induce metamorphosis of planula larvae into adult polyps. We report here that RFamides act antagonistically to LWamides. RFamides inhibit metamorphosis when applied to planula larvae during metamorphosis induction by treatment with LWamides (or other inducing agents such as CsCl ions, diacylglycerol and bacterial inducers). Our results show further that RFamides act downstream of LWamide release, presumably directly on target cells mediating metamorphosis. These observations support a model in which metamorphosis in H. echinata is regulated by sensory neurons secreting LWamides and RFamides in response to environmental cues.Edited by D. Tautz     

Priority effects of the early breeding fire salamander on the late breeding banded newt

Early breeding intraguild predators may have advantages over late breeding predators via priority effects; early breeding predators may reduce shared prey resources before late breeders appear and may also prey upon the late breeders. Here we show that predatory larvae of the late-breeding predatory banded newt, Triturus vittatus vittatus, occupy the same temporary pond toward the end of the developmental period of the early-breeding predatory fire salamander, Salamandra salamandra, resulting in a large size disparity between larvae of these two species while they co-occur. We conducted outdoor artificial pool experiments to assess priority effects of large larval Salamandra at the end of their larval development period, on recently hatched larval Triturus. We also assessed how artificial vegetation may influence larval Triturus performance in the presence or absence of Salamandra Salamandra, introduced into the experimental pools two weeks prior to the newt larvae, strongly reduced invertebrate prey abundance shared by these two predatory urodeles and with only a one week period of overlap, strongly reduced abundance of Triturus larvae. The artificial vegetation had only a small ameliorating effect on Triturus survival when Salamandra was present. Triturus size at metamorphosis (snout-tail length) was significantly larger in the Salamandra pools, presumably due to a combination of a strong “thinning effect” and greater vulnerability of smaller Triturus individuals to predation by Salamandra. Time to metamorphosis was not significantly affected by Salamandra. These results have conservation implications as T. v. vittatus is listed as highly endangered and may also explain the largely negative spatial association of the two species. Handling editor: K. Martens     

Inshore migration,seasonal distribution and sizes of larval bonefish,Albula, in the Gulf of California

Synopsis The leptocephalous larval stages of the bonefish (Albulidae: Albula) are abundant in the coastal regions and hypersaline lagoons (esteros) throughout the Gulf of California during winter and spring. Inshore movements of leptocephali were studied by sampling the entrance to Estero Miramar, at Guaymas, Sonora, Mexico, from February to May, 1981. Thirty-three samples were taken at various times during flood tide. All but four of the samples were taken after sunset. Leptocephali were collected in all months except May. The results suggest that leptocephali enter the estero only during the first few hours of flood tide. Large numbers of larvae were observed during the day at ebb tide maintaining their near-shore position in the entrance channel. These larvae did not appear to be migrating into the estero. This, together with the absence of larvae in daytime samples, suggests that the migration is nocturnal. Most of the 667 leptocephali collected during the sampling period had completed larval growth or were just beginning the metamorphic interval. The median standard length (SL) was 61 mm and the mode was 62 mm. The largest leptocephali measured 71 mm SL. Since metamorphosis is accompanied by a decrease in length, it was concluded that the maximum size attained during larval growth is about 71 mm SL.     

Histological changes in the pituitary-thyroid axis during spontaneous and artificially-induced metamorphosis of larvae of the flounder Paralichthys olivaceus

Summary Histological changes in the pituitary TSH cells and in the thyroid gland of flounder (Paralichthys olivaceus) larvae during spontaneous or artificially induced metamorphosis were studied. Activity of the immunoreactive TSH cells (IrTSH cells) gradually increased during premetamorphosis, reaching the highest level in prometamorphic larvae, and the cells were degranulated in metamorphic climax. The IrTSH cells were most inactive at the post-climax stage. The thyroid gland was morphologically the most active in metamorphic climax when the degranulation occurred in the pituitary IrTSH cells, and appeared inactive at post-climax. A few weeks after metamorphosis, both the IrTSH cells and the thyroid gland appeared to be activated again in the benthic, juvenile flounder. Administration of thyroxine or thiourea revealed negative feedback regulation of the pituitary-thyroid axis in flounder larvae. These results indicate that activation of the pituitary-thyroid axis induces metamorphosis in the flounder.     

Investigations on the thyroid gland of embryonic larval and adult Ichthyophis glutinosus and Ichthyophis kohtaoensis (Gymnophiona,amphibia)

Summary Different developmental stages of two species of the genus Ichthyophis have been investigated. In the late embryo the follicular cells of the thyroid gland exhibit various degrees of cytodifferentiation. Well differentiated cells show a polar organization and contain numerous granular inclusions, but a colloid-containing lumen is rare. Most cells at this stage contain large lipid inclusions. In young and older larvae the cells contain well-developed rough ER and Golgi systems, numerous mitochondria, and abundant granular and vesicular inclusions. Tentative identifications were made of primary lysosomes, secondary lysosomes, residual bodies, and two types of small apical vesicles—containing resorbed colloid or transporting material into the follicular lumen. In the larvae the number of apical microvilli is relatively high. The thyroid cells of the older larvae seem to contain more granular and vesicular inclusions than those of the younger larvae. In the adult the size of the follicles greatly increases, the height of the epithelium decreases, microvilli become rare, residual bodies are more frequent, and the small primary lysosomes are replaced by larger ones. Colloid droplets have been found only rarely in the cytoplasm of the thyroid cells of adult animals. In the immediate neighbourhood of the follicular epithelium, profiles of nerve fibres were found in all animals. Radioiodide investigations—measurements of conversion ratio and thyroid uptake factor—show, if compared with the results of corresponding studies in other amphibians, only relatively small differences between the larvae on the one hand and larvae and adults on the other. The absolute counts of the thyroid region are lowest in the adult and highest in the older larvae, shortly before metamorphosis. Furthermore our results indicate, on the basis of four animals tested, that in Ichthyophis the activity of the thyroid gland is temperature dependent. The results in Ichthyophis show that the classical stages of metamorphosis, in other amphibians characterized among other things by different levels of thyroid activity, are very indistinct in this animal.We gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft (We 380/5; Sto 76/4).     

Premetamorphic bonefish (Albula sp.) leptocephali from the Gulf of California with comments on life history

Synopsis Ten early premetamorphic bonefish (Albula sp.) leptocephali were collected in MOCNESS plankton tows in the Guaymas Basin area of the Gulf of California from 27 July–1 August, 1985. Initial captures of five larvae showed that these were found only in surface waters (0–100 m) at temperatures of 15–29° C. A shallow tow made from 200 m to the surface resulted in the capture of five additional larvae that were distributed in the upper 50 m of the water column at temperatures of 21–29° C. Both pre-flexion larvae (<17.0 mm notochord length, NL) and larvae with flexed notochords were captured. The size range was from 12.0 mm NL to 27.0 mm standard length. Drawings of representative larvae are given. This is apparently the first report of premetamorphic bonefish larvae for the Gulf, although metamorphic larvae have been known to be abundant in coastal areas of this region for almost 100 years. Distribution records for metamorphic larvae are reviewed. This, together with observations on seasonal distribution of ripe adults in coastal waters near Guaymas, Sonora, Mexico, has allowed us to speculate on various aspects of life history. Our data suggest that spawning occurs during late spring and summer and that early development takes place offshore. The premetamorphic interval is postulated to last approximately 6–7 months. Larvae then return to coastal areas during the winter and spring to complete metamorphosis.     

Substance P- and cholecytokinin-like immunoreactivity during post-metamorphic development of the central nervous system in the ascidian Ciona intestinalis

Following metamorphosis, the neural ganglion of ascidians is thought to be formed via the proliferation of epithelial cells comprising the ciliated duct. In adults, neuronal cell bodies expressing substance P- and gastrin/cholecystokinin-like immunoreactivity exhibit clearly defined patterns of distribution. Previous work shows that these patterns are re-established during regeneration of the adult ganglion. We have used antisera against substance P and cholecystokinin to monitor the formation of these patterns during normal post metamorphic development in Ciona intestinalis. Substance P cells first appear in the ganglion in animals of 1 mm body length. Cholecystokinin antiserum was not used at this stage but revealed a clear adult-like pattern of cells in the anterior region at the 3 to 5-mm stage. Substance P cells do not exhibit an adult pattern until animals have a body length of more than 10 mm. Proliferation in the neural complex was studied using the bromodeoxyuridine/anti-bromodeoxyuridine technique. Results suggest a mechanism whereby cells are born in the ciliated duct and later migrate to the ganglion. Double-labelling experiments indicate that more than 11 days elapse between cell birthdates and the expression of either of the peptides. Data presented suggest that the distributional patterns for these peptides during normal development are similar to those seen during regeneration.     

Growth and population structure of Salamandra salamandra (L.) larvae in different limnological conditions

Salamandra salamandra (L.) larvae were studied in three types of aquatic systems, a permanent spring, a rock pool and temporary ponds. Growth was followed throughout ontogenesis until completion of metamorphosis. Growth patterns changed according to the pond type. Temporary ponds were rich in organic matter and subsequently larvae matured more rapidly there than those in a permanent pool or a spring. In the latter, cannibalism appears to be of major ecological importance, enabling survival and metamorphosis of large size tadpoles. The pattern of growth and metamorphosis is compared with European salamanders and the differences are discussed considering adaptations for survival of this species under xeric conditions.     

Predicting larval metamorphosis of Pacific lamprey Entosphenus tridentatus through measurements of length and mass

The utility of length and mass measurements to predict the larval metamorphosis of Pacific lamprey Entosphenus tridentatus was evaluated. During 2004–2008, larval E. tridentatus were collected from Cedar Creek (Washington, USA) in either the spring or autumn, measured for total length and total mass, reared in captivity and monitored for metamorphosis. The minimum total length, total mass and condition factor of larvae that were observed to go through metamorphosis were 102 mm, 2.0 g and 1.52, respectively. Logistic models indicated that total length and condition factor in both spring and autumn were the most significant variables for predicting metamorphosis of Pacific lamprey during the subsequent summer. Mass in the autumn also appeared important to predict whether metamorphosis occurred in the subsequent summer. Collectively, all models using specific minimums of total length, total mass or condition factor of larvae as criteria for them to metamorphose were sometimes (5 of 14 cases) able to predict the percentage of larvae that would metamorphose but rarely (1 of 12 cases) able to predict which individual larvae would metamorphose. Similar to other anadromous species of lampreys, the size and condition of larval E. tridentatus have utility for predicting metamorphic fate.     

Growth and development of larvae and galls of Urophora cardui (Diptera,Tephritidae) on Cirsium arvense (Compositae)

Summary The tephritid fly Urophora cardui induces a large multi-chambered gall within the stems of Cirsium arvense. Three distinct phases of gall development have been identified as initiation, growth, and maturation. During initiation the insect gains control of tissue development and during the gall's growth phase parenchyma cells proliferate rapidly surrounding the larvae with thick layers of cells. Patches of primary nutritive cells appear along the surface of larval chambers during the growth phase but few of these cells are consumed. In the gall's maturation phase, thick layers of secondary nutritive cells appear around the surface of larval chambers and the remaining gall parenchyma lignifies. Secondary nutritive cells are the primary food of U. cardui.The gall expands rapidly during the growth phase then abruptly slows at the beginning of the maturation phase. Rate of gall growth is dependent upon the number of larvae per gall but the number of larvae does not affect duration of this phase.Larvae remain in the second instar throughout the growth phase and grow slowly. Once the gall enters the maturation phase and the secondary nutritive cells appear, the larvae moult to the third instar and grow quickly. Larvace attain over 98% of their final weight during the maturation phase and consume all secondary nutritive cells.It is postulated that larvae do not feed extensively on primary nutritive cells since these cells play a key role in gall morphogenesis. The appearance of secondary nutritive cells stimulates larval feeding at a time when gall growth and development is finished.     

Influences of host size and host species on theinfectivity and development of Heterorhabditismegidis (strain NLH-E87.3)

The infectivity, time to first emergence of infective juveniles (IJs), total number of IJs per insect and IJs body length of the entomopathogenic nematode Heterorhabditis megidis (strain NLH-E87.3) after development in larvae of two insect hosts, Galleria mellonella (greater wax moth) and Otiorhynchus sulcatus (vine weevil) was studied. At a dose of 30 IJs, larvae of G. mellonella show to be significantly more susceptible than O. sulcatus larvae. At a dose of one IJ, vine weevil larvae were more susceptible. The number of invading infective juveniles (IJs) increased with host size while the host mortality at a dose of one IJ decreased with the increase of host size. Time to first emergence was longer at a dose of one IJ per larva and increased with the increase of host size in both insect species. Reproduction of IJs differed between host species, host sizes and doses of nematodes. Generally, the IJs body size increased with an increasing host size. The longest infective juveniles were produced at the lowest IJ doses. Results are discussed in relation to the influence of different host species and their different sizes on the performance of H. megidis (strain NLH-E87.3) as a biological control agent.     

Structure of the femora and autotomous (postpygal) caudal vertebrae in the three-toed skink Chalcides chalcides (Reptilia: Squamata: Scincidae) and its applicability for age and growth rate determination

The limbs of the three-toed skink Chalcides chalcides are reduced to such a degree that the three digits are too small for skeletochronology. This study, performed on animals collected near Isernia (central Italy), describes the structure of the caudal vertebrae, which are often naturally lost due to autotomy, in order to determine whether they can be used to obtain data on age and growth with skeletochronological techniques. The reliability of the autotomous caudal vertebrae for skeletochronology was verified by performing skeletochronological analyses also on femora. Although the identification of the lines of arrested growth (LAGs) was easier in femora than in autotomous caudal vertebrae, a high correspondence of the LAG count between the two bones was observed. Females were larger and lived longer than males (4 and 3 years, respectively). For both sexes, the snout vent length (SVL) was significantly correlated with age. For both sexes, sexual maturity was attained after two hibernations from birth, beginning at the 20th month of age. At first reproduction, males had an SVL of 91–106 mm and females one of 111–150 mm. Von Bertalanffy growth curves of age versus SVL showed that females had slower growth rates than males for attaining their asymptotic SVL (females: 197 mm; males: 143 mm). The results provide the first data on age and growth of C. chalcide, and show that autotomous caudal vertebrae are reliable alternatives for obtaining such data for limb-reduced reptiles, avoiding the need to sacrifice or disable animals, as occurs when long bones are used.     

Responses of Mamestra brassicae (Lepidoptera: Noctuidae) to crowding: interactions with disease resistance,colour phase and growth

This study examines phenotypic plasticity in relation to rearing density in larvae of the moth, Mamestra brassicae. Larval phase, growth rate, weight at moulting and susceptibility to disease were quantified when reared at five densities. Larvae develop more quickly, but attain a smaller size and are more susceptible to disease, when reared at high than at intermediate densities. They also exhibit a higher degree of melanisation than larvae reared at intermediate densities, or singly. A review of the literature suggests that a switch to a rapidly developing dark phase at high densities is a widespread phenomenon within the Lepidoptera. Rapid development at the expense of attaining a large size, and increased melanisation, are interpreted as adaptive responses to reach pupation before food supplies are depleted, as is likely when larval density is high. High susceptibility to viral infection at high density may be a result of physiological stress associated with rapid development, or due to a shift in allocation of resources from resistance to development: larvae that developed quickly were more susceptible to infection. Larvae reared singly appeared to be less fit than larvae reared at intermediate densities: they exhibited many of the characteristics of larvae reared at high density, particularly low weight, a right-hand skew in their weight frequency distribution, and high susceptibility to disease. I hypothesise that expression of resistance may be phenotypically plastic with regard to environment. Contact with other larvae may, up to a point, stimulate both growth and resistance to infection, for the risk of infection will increase with the density of conspecifics.