Group size of feeding stream case-bearing caddisfly grazers and resource abundance

Several herbivorous insects are known to benefit from feeding in groups; however, little is known about (1) the resource conditions under which herbivorous insects perform group feeding and (2) the optimum population size to get any benefits by group feeding, for example, in terms of growth performance. To test the hypotheses that the benefits from group feeding change with resource level and population size, we performed field investigations and an enclosure experiment using the grazer caddisfly larva Micrasema quadriloba. The field investigations revealed aggregated distributions of larvae (indicator of aggregation, I_δ=4.1±1.55, aggregated density: 12.7±5.3 individuals per 3.1×3.1 cm² (mean±1 SD)) when periphyton was abundant on stream cobbles and random distributions (I_δ=1.0±0.11) when periphyton was scarce. In the enclosure experiment, the relative growth rate (RGR) of the larvae at each population size showed different tendencies at high and low periphyton abundance levels; RGR with abundant periphyton had a convex curve with a peak at intermediate population size, whereas RGR with scarce periphyton decreased linearly with increasing population size. The benefits from group feeding thus changed with resource level; larvae obtained high growth performance by group feeding behavior only when the resource was sufficiently abundant. The present study revealed not only that the optimum group size of larvae increased their growth performance, but also that this optimum group size occurred frequently in the field. We also discuss the mechanisms and benefits of group feeding by case-bearing caddisfly grazers.     

Un difficile problème de taxonomie : les Micrasema (Trichoptera : Brachycentridae) des eaux courantes de la Péninsule Ibérique et des Pyrénées

Un des problèmes taxonomiques les plus compliqués concernant les Trichoptères européens est celui de la plupart des espèces ibériques et pyrénéennes du genre Micrasema, habitants caractéristiques du Rhitral, ces espèces restant à ce jour imparfaitement connues et difficiles à déterminer. L´étude d’un grand nombre d´exemplaires (mâles) a permis de trouver une solution pour la plupart des problèmes. M. servatum (Navás 1918), distribuée dans les parties N.O. de la Péninsule; M. cenerentola Schmid 1952, connue du Centre et du N.O. de l´Espagne; et M. salardum Schmid 1952, connue des Pyrénées Orientales espagnoles, sont des espèces distinctes. M. gabusi Schmid 1952 décrite de la Sierra Nevada, est un synonyme de M. longulum McLachlan 1876, une espèce largement distribuée en Espagne. M. difficile Mosely 1933, décrite des Pyrénées françaises, est un synonyme de M. vestitum Navás 1918, espèce endémique des Pyrénées Centrales et Occidentales (France et Espagne). L´identité de M. moestum (Hagen 1868) pose un problème particulier : il est possible que sous ce nom on désigne actuellement une paire d´espèces cryptiques à l´état adulte, qu´il est pratiquement impossible de distinguer par leurs genitalia, mais dont les larves sont bien distinctes.     

Nanocladius (Plecopteracoluthus) asiaticus sp. n. (Diptera: Chironomidae) Phoretic on Dobsonfly and Fishfly Larvae (Megaloptera: Corydalidae)

The adult male, adult female, pupa, and larva of Nanocladius (Plecopteracoluthus) asiaticus sp. n. (Diptera: Chironomidae) are described and illustrated. Larvae live in coarse silken nets which usually are attached to the ventral side of the mesothorax of larval dobsonflies and fishflies (Megaloptera: Corydalidae) dwelling in stream riffles. The host species are Protohermes grandis and Parachauliodes continentalis in Japan, Protohermes costalis and Neochauliodes sinensis in Taiwan, and Neurhermes maculipennis in the Malay Peninsula. Larvae are not parasitic but may have a commensal relationship with their hosts; they feed on algae and detrital material caught on their silken nets, or on the body surface of their hosts. Pupation occurs in the fine silken net fixed to the lateral side of host thoracic regions, or occasionally to the undersides of hind legs.