The role of periphyton abundance and quality in the microdistribution of a stream grazer, Helicopsyche borealis (Trichoptera: Helicopsychidae)

SUMMARY. 1. A series of laboratory and field experiments was performed to determine if the preference of larvae of Helicopsyche borealis (Trichoptera: Helicopsychidae) for exposed rock surfaces in streams was related to patchiness of periphyton food.
2. At low current velocities in the laboratory larvae preferred food-saturated as opposed to food-depleted portions of substrata and drifted off the substratum in response to low food levels.
3. When offered a choice of three potential foods—a diatom, a green filamentous or a blue-green filamentous alga—larvae showed no preference.
4. Larval densities on artificial subtrata in field experiments were highest in areas of high algal chlorophyll a .
5. In laboratory experiments more adults emerged from cultures containing green algae or diatoms than cultures with blue-green algae or bacteria alone.
6. Periphyton quality was not as important as abundance in controlling H. borealis microdistribution. By selecting exposed surfaces of rocks, H. borealis larvae place themselves in the microhabitat most likely to contain both abundant and high quality food patches.     

Influence of plants on redox potential and methane production in water-saturated soil

Three chironomid midge (Diptera: Chironomidae) species (Cricotopus sylvestris (Fabricius); Psectrocladius limbatellus (Holmgren), Tanytarsus fimbriatus Reiss and Fittkau) live at high densities and dominate the midge assemblage found in slow sand filter beds. After filter beds are drained and cleaned, the substratum consists of sand of different particle sizes, but once beds are refilled and in operation, organic material accumulates on the sand and Cladophora grows. In laboratory experiments, we found that larvae of the three midge species preferred different substratum types: C. sylvestris preferred sand to both Cladophora and organic material, P. limbatellus preferred Cladophora, and Tanytarsus sp. preferred organic material. Some evidence of preference for one particle size was also found, but compared with substratum types, this was neither as marked or contrasting between the three species. The contrasting preferences that we found in the laboratory suggest that the substratum will influence the microdistribution of the three species in filter beds, and provide further evidence of the importance of substratum in midge assemblages.     

Factors influencing the attachment strength of Dreissena polymorpha (Bivalvia)

Abstract

The effects of several factors (shell length, exposure time, substratum orientation in space, illumination, temperature, conspecifics) upon the attachment strength (measured with a digital dynamometer) of the freshwater, gregarious bivalve Dreissena polymorpha were studied under laboratory conditions. A rapid increase in attachment strength was observed on resocart (a thermosetting polymer based on phenol-formaldehyde resin, with paper as filler) substrata during the first 4-d exposure, after which it stabilised at ca 1 N. The attachment strength increased also with mussel size. Mussel adhesion on variously oriented surfaces (vertical, upper horizontal and lower horizontal) was similar. Illumination inhibited attachment strength, as expected for a photophobic species, but only after a 2-d exposure. After 6 d, no effects of light were detected. Thus, illumination seemed to influence the attachment rate, rather than the final strength. The optimum temperature for mussel attachment was 20 – 25°C. At lower and higher temperatures (5 – 15°C and 30°C), their adhesion strength decreased. The presence of conspecifics stimulated mussel attachment strength.     

The effect of substrate rearing on growth,aerobic scope and physiology of larval white sturgeon Acipenser transmontanus

The effect of substratum on growth and metabolic rate was assessed in larval white sturgeon Acipenser transmontanus. Yolk‐sac larvae (YSL) were reared in bare tanks or tanks with gravel as substratum from hatch until approximately 16 days post hatch (dph). The effect of an artificial substratum was also evaluated on growth alone. Substratum had a significant effect on mass, with larvae reared in gravel and artificial substrata being larger than those reared without substratum. Routine metabolic rates were significantly lower and relative aerobic scope (the difference between maximum and routine metabolic rate) was significantly higher for YSL and feeding larvae (FL) reared in gravel relative to those reared in bare tanks, particularly before fish started feeding exogenously. Furthermore, gravel‐reared larvae had higher whole‐body glycogen concentrations relative to bare‐tank‐reared larvae. Routine factorial scope (maximum metabolic rate divided by routine metabolic rate) was relatively low in all treatments (< 1·7) indicating a limited ability to elevate metabolic rate above routine early in development and mass exponents for metabolic rate exceeded 1. Taken together, these data indicate that YSL reared without substratum may divert more of their energy to non‐growth related processes impairing growth. This finding underscores the importance of adequate rearing substratum for growth of A. transmontanus and may provide support for habitat restoration and alternative hatchery rearing methods associated with sturgeon conservation.     

Diversity, distribution and larval habitats of North Swedish blackflies (Diptera: Simuliidae)

1. Blackfly species richness and community structure were analysed at fifty-six sites in northern Sweden in two seasons. The sites were situated in a wide range of streams and rivers from small springbrooks, bog streams and lake-outlet streams to medium-sized forest rivers and large rivers draining montane regions.
2. Thirty-nine blackfly species were found, with between two and thirteen species per site. Neither species richness nor abundance could be related to the environmental variables measured.
3. An analysis of labral fan size of blackflies indicated a clear trend for the prevalence of larvae with small fans in large rivers and larvae with larger fan size in small streams. Similarly, fan size related to current velocities so that large fans were associated with slow current velocities and small fans with high velocities.
4. A strong relationship existed between species composition and habitat, as seen in ordination by non-metric multidimensional scaling. The relationship found between fan size and habitat size-related variables, such as channel width, depth, velocity and substratum particle size, along with longitude and altitude, in partial least squares regression analysis offered an explanation of the species composition–habitat relationship.
5. In addition to testing that distributions of blackfly larvae reflect morphological traits, we tested two general hypotheses pertaining to distribution patterns: (a) that blackfly communities show bimodal distributions; and (b) that their distributions are nested. Neither of these two hypotheses was supported by our observations. However, widespread blackfly species were locally more abundant than those found at relatively few sites, thus showing a positive abundance–occupancy relationship.     

The effect of substratum type on aspects of swimming performance and behaviour in shortnose sturgeon Acipenser brevirostrum

The swimming performance and associated swimming behaviour (i.e. substratum‐skimming, station‐holding and free swimming) were assessed in shortnose sturgeon Acipenser brevirostrum during critical swimming and endurance swimming tests over a rough and a smooth substratum. It was hypothesized that the addition of a rough substratum in the swimming flume may provide a surface for the A. brevirostrum to grip and offer an energetic advantage. Substratum type did not affect the critical swimming performance, but A. brevirostrum consistently performed more bottom behaviours (i.e. substratum‐skimming and station‐holding) while on a smooth substratum. Acipenser brevirostrum had little contact with the rough substratum until the velocity was >1 body length s^?1. Endurance swimming time was significantly lower for A. brevirostrum over the rough bottom at the highest velocity (30 cm s^?1) which may be attributed to the observed increase in free swimming and decrease in bottom behaviours. During endurance swimming, the rough substratum was mainly used at intermediate velocities, suggesting that there may be a stability cost associated with being in contact with the rough substratum at certain velocities.     

Day–night changes in the spatial distribution and habitat preferences of freshwater shrimps, Gammarus pulex, in a stony stream

1. As many invertebrates are nocturnal, their spatial distribution and habitat preferences may change from day to night. Both aspects are examined for Gammarus pulex by testing the hypotheses: (i) a power function was a suitable model for the spatial distribution of the shrimps in both day and night; (ii) diurnal and nocturnal spatial distributions were significantly different; (iii) diurnal and nocturnal habitat preferences were significantly different. Five different life‐stages were treated separately. To ensure that the conclusions were consistent, large samples were taken near midday and midnight in April, June and November over 4 years at two sites about 3 km apart in a stony stream: downstream (n = 30) and upstream (n = 50). 2. The first and second hypotheses were supported at both sites. A power function, relating spatial variance (s²) to mean (m), was an excellent fit in all analyses (P < 0.001, r² > 0.91), i.e. the spatial variance was density‐dependent. All five life‐stages were aggregated in the day. At night, the degree of aggregation increased for juveniles at higher densities but decreased for juveniles at lower densities, increased for immature females and males, but decreased slightly for mature females and especially mature males, the latter being close to a random distribution. There were no significant differences between sites, in spite of the lower numbers at the downstream site. 3. The third hypothesis was tested at only the upstream site and supported by comparisons between shrimp densities and 13 physical variables (distance from bank, water depth, water velocity, ten particle size‐classes), and three non‐physical variables (dry weights of bryophytes, leaf material, organic detritus). During the day, densities were strongly related to particle sizes with the following preferences: 0.5–8 mm for juveniles, 8–256 mm for the other life‐stages with a weaker relationship for males. There were no significant positive relationships with the other variables, apart from bryophytes for immature shrimps and adults. At night, densities were unrelated to particle size; juveniles and immature shrimps preferred low water velocities near the banks, often where leaf material and organic detritus accumulated, females often preferred medium water velocities slightly away from the banks, and males showed no habitat preferences. 4. Day samples do not provide a complete picture of habitat preferences and probably identify refuge habitats. Day–night changes in spatial distribution and habitat preferences are an essential part of the behavioural dynamics of the shrimps and should be investigated in other species.     

Microhabitat and plant structure of Batrachospermum (Batrachospermales, Rhodophyta) populations in four streams of São Paulo State, southeastern Brazil

Microhabitat and plant structure of seven Batrachospermum populations (four of Batrachospermum delicatulum (=Sirodotia delicatula), one of Batrachospermum macrosporum and two of the ‘Chantransia’ stage), including the influence of physical variables (current velocity, depth, irradiance and substratum), were investigated in four streams of São Paulo State, southeastern Brazil. The populations of B. delicatulum and the ‘Chantransia’ stage occurred under very diverse microhabitat conditions, which probably contributes to their wide spatial and seasonal distribution in Brazilian streams. Results suggest branch reconfiguration as a probable mechanism of adaptation to current velocity based on the occurrence of: (i) B. macrosporum (a large mucilaginous form with presumably little ability for branch reconfiguration) under lower current velocity than B. delicatulum; (ii) only dense plants in populations with high current velocities (> 60 cm s^?1), whereas 53–77% of dense plants were seen in populations exposed to lower currents (< 40 cm s^?1); (iii) positive correlations of plant length with internode length in populations under low current velocities and negative correlation in a population with high velocity (132 cm s^?1); and (iv) negative correlations of current velocity with plant diameter and internode length in a population under high flow. This study, involving mainly dioecious populations, revealed that B. delicatulum displayed higher fertilization rates than B. macrosporum. A complementary explanation for a dioecious species to increase fertilization success was proposed consisting of outcrossing among intermingled male and female adjacent plants within an algal spot.     

Habitat preferences of Ukrainian brook lamprey Eudontomyzon mariae ammocoetes in the lowland rivers of Central Europe

The pattern of microhabitat preferences of Ukrainian brook lamprey Eudontomyzon mariae ammocoetes was examined in two rivers of central Poland: the Pilica River (the Vistula River basin) and the Grabia River (the Odra River basin). A comparison of abiotic factors of the rivers revealed differences in water speed and principal components: PC1 (determining gradient from decreasing medium sand to the increasing share of three fractions of gravel), PC2 (a gradient from the declining share of very coarse and coarse sand fractions to the growing content of fine sand) and PC3 (correlated with an increasing proportion of silt). The sites did not differ significantly in terms of water depth. Relative abundance and frequency of ammocoete occurrence in the Grabia River were higher than in the Pilica River. Only speed, PC1 and PC2 made a significant contribution to the prediction of larval occurrence. Eudontomyzon mariae larvae preferred substrata with a reduced amount of medium sand and increased content of gravel (PC1) as well as with a lower content of coarse sand and higher proportion of fine‐grained sand (PC2). The ammocoetes also preferred areas with a water speed of 0·2 m s^?1 but avoided speeds ≥0·6 m s^?1. The abundance of E. mariae was affected by water speed, as well as by all PCs. The mean ± s.e. optimal current speed was 0·265 ± 0·007 m s^?1, while abundance decreased with increasing amounts of gravel (PC1) and increased with increasing amounts of fine sand and silt in the bottom substratum (PC2 and PC3). Comparison of ammocoete microhabitat use in the Pilica and Grabia Rivers showed the lack of differences in distribution in the preferred values of current speed, PC1 and PC2.     

Seawater DMS in a perturbed coastal ecosystem

Intra- and interspecific niche overlap for two mayfly species with similar life cycle timing, Rhithrogena semicolorata and Ecdyonurus sp. gr. venosus, were investigated. The nymphs were classified into 5 classes according to size and spatial overlaps are measured along a substratum roughness gradient. Substratum roughness selection was investigated by defining utilisation curves, optimum and tolerance values of the nymphs in relation to larval growth. Differences between species and size classes within each species were observed. Ecdyonurus sp. gr. venosus dominated on rough substrates, whereas R. semicolorata was most abundant on smooth substrates. An intermediate value of total interspecific substratum roughness overlap (0.49) was found. Higher intraspecific than interspecific overlap values suggested a spatial niche segregation between the two species. The results suggested that the spatial niches measured, were closer to the `fundamental niches' than could be expected if competition was acting on the two studied populations.     

The reaction of larvae of Simulium noelleri (Diptera) to different current velocities

Larvae of Simulium noelleri Friederichs aggregate at high population densities (more than 10² cm^–2) on sluices, dams, and spillways. Experiments were conducted in a laboratory trough to assess the reaction of larvae to different current velocities (velocities ranged from 5–49 cm s^–1). In the lower part of the range of water velocities used, larvae moved a greater distance upstream from where they had been located. Larger larvae always showed a greater tendency to move than did smaller larvae, whatever the velocity. This intraspecific variation in reaction to different current velocities allows the aggregation of larvae of mixed sizes at suitable sites, smaller individuals being occluded by those that are larger.     

Colonization of natural substrata of different roughness and colour by Ephemeroptera nymphs using retrieval and direct observation techniques

We investigated the colonization of stream substrata by Ephemeroptera nymphs in a Canadian Rocky Mountain stream. Stream substrata used in our study were different textures (smooth and rough) and colours (dark and light); the taxonomic groups studied were Drunella coloradensis Dodds (Ephemerellidae), Heptageniidae (Cinygmula and Epeorus) and a Baetis Ameletus group (Baetidae and Siphlonuridae). We tested the hypothesis that as D. coloradensis nymphs and other abundant Ephemeroptera nymphs increase in size they choose a dark substratum to remain cryptic and a substratum of greater texture to reduce the risk of being swept from the substratum by the water flow. Two approaches were used to test this hypothesis, a substratum basket experiment and direct observations.For the substratum basket experiment, there were no statistical differences in abundance of nymphs on the substratum types. But there was a general tendency that the D. coloradensis nymphs were least abundant on the rough dark-coloured substratum and developmental stage three nymphs were most abundant on the smooth white-coloured substratum.Colonization was investigated for other Ephemeroptera nymphs by making direct observations on the substratum baskets using a glass-bottomed box. For the taxonomic groups and two colonization periods, nymphs were statistically at greatest density on the smooth white-coloured substratum and lowest on the smooth black-coloured substratum. The Baetis-Ameletus groups were also found at greatest densities on the rough dark-coloured substratum.     

Competition in Bembicium auratum (Gastropoda) and its effect on microalgal standing stock in mangrove muds

Summary Field experiments were undertaken on the intertidal gastropod Bembicium auratum, which is one of the dominant organisms in sheltered bays and mangroves in New South Wales. Animals were caged at various densities ranging from natural levels to about four times normal density. Increased density resulted in increased mortality (particularly of juveniles), reduced body weight, and (less conclusively) a decline in growth rate. Chlorophyll levels of the mangrove mud within the cages were monitored as an index of food availability, and increased rapidly in the control cages in the absence of Bembicium, remained steady at normal Bembicium densities, and declined at higher densities. Parallel experiments were conducted on the effects of substratum on Bembicium. One series of animals was allowed access to a hard substratum and the other only to mud. Body weights and survivorship were proportionally lower in animals deprived of a hard substratum. Juveniles were particularly susceptible to a shortage of food and more dependent on a hard substratum than adults. The population structure also suggests a high juvenile mortality but high adult survival and longevity. Bembicium is very abundant low on the shore but numbers decline and body weights increase higher up. Chlorophyll levels and amounts of hard substratum also decline up the shore. Possibly juveniles settle selectively (or survive better) low on the shore where oysters provide a dense hard substratum. Adults are less dependent on the hard substratum and may disperse up the shore, explaining the patterns of abundance and size.As Bembicium limits its own food supply and seemingly is limited by it, predation is unlikely to play an important role in restricting population density.     

Changing risk of predation for a filter-feeding insect along a current velocity gradient

Flume experiments were carried out to examine whether larval blackflies (Simulium ornatum complex, Diptera: Simuliidae) use microhabitats with a highvelocity current to reduce the risk of predation by some of their main predators, viz. larvae of the stoneflies Isoperla grammatica and Diura nanseni (Plecoptera: Perlodidae), and the caddis-fly Rhyacophila nubila (Trichoptera: Rhyacophilidae). We exposed blackfly larvae to four different current velocities and measured their feeding rate using dye particles. The maximum feeding rate was recorded at intermediate velocities (18.8 and 36.2 cm/s), whereas at low (7.3 cm/s) and high (53.3 cm/s) velocities, the feeding rate was reduced. In separate experiments, we investigated the behaviour and attack success of the different predator species. The two perlodids showed a similar hunting behaviour, which was significantly less successful at higher velocities. Drift of the perlodids from the experimental arena resulted in reduced encounter and attack rates, especially in I. grammatica, which had completely lost efficiency at 36.2 cm/s. R. nubila had a slower mode of hunting and was unaffected by current speed within the velocity gradient studied. Drift in Rhyacophila was rare. Observations on the behaviour of blackfly larvae were performed in the same experiments. The larvae showed no apparent ability to sense the presence of the predators except when these disrupted the flow pattern or were in physical contact, which often resulted in aggressive defence, though without effect on the predators. Escape of blackfly larvae by drift did occur, but this was no more common than being captured. In a current velocity gradient, blackfly larvae showed a weak preference for increasing velocities. Thus, at velocities between 7 and 54 cm/s, blackfly larvae appear to select microhabitats with high current velocities, despite a reduction in feeding optimality, thereby easing the predation impact from perlodids, though not from Rhyacophila. The study demonstrates the importance of microhabitat selection by blackfly larvae both for efficient feeding and predator avoidance.     

Experiments on predation and substratum choice by larvae of the muscid fly, Limnophora riparia

Larvae of the muscid fly Limnophora riparia live in lake outlets and prey on other invertebrates living there. In experiments, we demonstrated that larvae prefer moss as a substratum, though they will bury themselves in any suitable material to avoid light. The substratum is used to anchor Limnophora larvae as they attack their prey. When given a choice of prey they preferred chironomid and black fly larvae to oligochaetes and psyehodid larvae. Larvae of the black fly Simulium noelleri were used in laboratory experiments to test the interaction of predator and prey. Limnophora larvae attacked black fly larvae of all sizes, but preferred small larvae, the body contents of which were often removed completely. Increasing prey or predator density did not affect this latter preference, though an increase in predator density, or a decrease in prey density, did cause the predator to take prey of medium size as well as small prey. Limnophora larvae showed the same size preference when attacking dead (freshly-killed) prey and they preferred to attack larvae rather than pupae when both were available. They did not attack black fly eggs.     

Estimation of preferred water flow parameters for four species of Simulium (Diptera: Simuliidae) in small clear streams in South Africa

Blackfly larvae typically occur in fast-flowing riffle sections of rivers, with different blackfly species showing preferences for different hydraulic conditions. Very little quantitative data exist on hydraulic conditions linked to the blackfly species occurring in South African streams. Stones-in-current biotopes (i.e. fast riffle flows over cobbles) were sampled from four sites in three small clear streams in the Eastern and Western Cape provinces of South Africa. Mean water column velocities at each sampled stone were measured using a mini current meter, while flow velocities closer to the boundary layer where blackfly larvae occurred were estimated using indirect techniques (standard hemispheres and aerating tablets). Standard hemispheres were also used to calculate more complex hydraulic parameters such as Froude and Reynolds numbers. Four species of Simuliid were sampled in sufficient numbers to show trends in flow velocity preferences. Simulium impukane and S. rutherfoordi both occurred at their highest densities at velocities of 0.3m s^?1, while S. merops preferred velocities of 0.7m s^?1. Simulium nigritarse SL attained the highest densities of all the blackfly species sampled, and its relative abundances were greatest at velocities of 0.8–0.9m s^?1. Within the streams surveyed, all blackfly species occurred in subcritical-turbulent flows — based on a classification using Froude and Reynolds numbers — although two of the species were also found in high densities in supercritical flows where these existed at the sites. Local hydraulics within the stones-in-current biotope are complex, but in the absence of fine-scale equipment for measuring micro-velocities, standard hemispheres are a useful, cost-effective technique for the initial quantification of hydraulic parameters in small, clear streams. Such an approach facilitates further understanding of links between hydraulics and aquatic invertebrates in South African streams.     

Interactive effects of shade and surface orientation on the recruitment of spirorbid polychaetes

A series of observations and an experiment were carried out to test hypotheses about the effects of shade on the densities of spirorbid polychaetes (Neodexiospira spp.) on intertidal pneumatophores (mangrove roots) of Avicennia marina. Densities of spirorbids were greater on pneumatophores surrounded by seagrass (Zostera mucronata) than patches without seagrass. Within patches of seagrass, the density and survivorship of spirorbids on pneumatophores was greater near the substratum (covered by seagrass) than high above the substratum (not covered by seagrass). The model that these patterns of abundance are explained by greater recruitment of spirorbids to shaded surfaces was assessed. This was done by experimentally testing the hypothesis that recruitment to patches without seagrass would not differ between the upper (unshaded) and lower surfaces (unshaded) of clear plastic sheets, but would be greater on the lower surfaces (shaded) than upper surfaces (unshaded) of black plastic sheets. Recruitment was consistent with these predictions and therefore provided evidence that differences in densities of spirorbids between substrata with and without seagrass may be due largely to differences in shading.     

Unsuitability of Cnesterodon decemmaculatus (Jenyns, 1842) for mosquito control in Uruguay: evidence from food‐preference experiments

We analyzed the feeding preference of Cnesterodon decemmaculatus, a small‐bodied poecilid native from the Rio de la Plata and proximate Atlantic Basins in South America. This species has a wide distribution in Uruguayan water bodies but its effectiveness as a predator of mosquito larvae has not been tested. In laboratory trials, five aquatic invertebrates were offered simultaneously as potential prey to fish: Daphnia pulex (Cladocera), copepods, two different instars of mosquito larvae (Culex pipiens), and the 4^th instar of Chironomidae larvae. Preference was measured by the Chesson's electivity index (α). In order to determine differences in prey preference according to fish size, individuals ranging from 9.5 mm to 35.3 mm were classified in three different body size classes: small, medium, and large. Small fish showed preference for copepods, while medium‐sized fish preferred the smallest mosquito larvae instars and Chironomidae larvae. We conclude that C. decemmaculatus is a zooplankton facultative‐feeder fish that prefers large‐bodied zooplankton but is a weak predator of mosquito larvae. Thus, the introduction of C. decemmaculatus as a biological‐control agent in natural environments is not an effective strategy.     

ÉTUDE AU LABORATOIRE DE LA RECHERCHE DU SITE DE TISSAGE DU COCON PAR LES LARVES DE CHRYSOPA PERLA (L.) (NEUROPTERA,CHRYSOPIDAE)

Résumé Les larves de Chrysopa perla lorsqu'elles sont prêtes à tisser leur cocon manifestent une tendance à descendre de leur support, recherchant secondairement untsite obscur et qui assure le contact. Lorsque le milieu est sect et meuble, elles s'enfouissent à une profondeur qui n'excède pas 20 mm dans la sciure. Les larves destinées à entrer en diapause s'enterrent plus profondément que celles qui ont un développement rapide continu. L'humidité excessive est un facteur répulsif énergique interdisant le plus souvent le tissage d'un cocon étanche. Lorsque le milieu ne permet pas l'enfouissement, les larves tissent successivement au contact les unes des autres, provoquant ainsi le regroupement des cocons.

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Summary The larvae of Ch. perla tended to leave their supports and look for a dark, thigmotactic place when ready to spin their cocoons (Table III). Darkness is not essential for spinning, successful pupation behaviour or for a low prenymphal mortality, under the light-intensity conditions used (3,000 lux). Under extremely wet conditions, the larvae did not easily find a place to spin their cocoons and may in fact return to their original support. In such conditions, the active larval stage for searching a suitable spinning place increased, the larva may die before a cocoon has been spun or an abnormal rough, porous, yellow cocoon may be made.Ideally a loose, dry substratum was chosen to spin the cocoon; the light substratum (sawdust and vermiculite) was always chosen in preference to the heavy one (earth and sand) and a dry substratum rather than a wet one (Table IV). In the light substratum, pupation took place in the upper 20 mm. In the heavy substratum, cocoons were spun on the surface and were adjacent to each other (Fig. 1) to allow support for the external threads. In both cases, substrate particles enclosed the cocoons.When the larvae were subjected to diapausing conditions by a short-day photoperiod, the mean weight of the cocoon was larger (14.21±1.83 mg) than if development was rapid and continuous (13.00±1.84 mg). Larvae which have been induced into diapause were found deeper in the sawdust than those which had not (Figs 2 and 3); but the difference was too small to be used as a method of separating the two populations.