TRICHOPTERA. — PHRYGANEIDA. 67 
the genus Mystacida, as above noticed, differ in having the posterior 
pair of legs greatly elongated ; whilst those of Sericostoma have the 
external organs of respiration short and united on a common base, so 
as to form small bundles of short filaments. In the larve of the Tricho- 
stome the anterior angles of the thoracic segments are greatly pro- 
duced in front, forming acute points; and in those of the Hydroptile, 
the thoracic segments are narrow, and the abdomen swollen and 
destitute of external organs of respiration ; these reside in small flat- 
tened kidney-shaped cases, opened by a slit at each end (fig. 68. 5. 
case of H. pulchricornis Pietet). M. Pictet further notices the 
interesting circumstance (long ago, however, observed by Willughby, 
as above detailed), that the larvee of many of these insects reside in 
immoveable cases attached to stones, &c., but formed of materials 
similar to the moveable cases* (fig. 68.6. case of Hydropsyche 
senex Pictet: fig. 68. 8. elongated tortuous fixed cases, composed of 
silk and fine sand, formed by the larvae of Hydropsyche maculicornis 
Pictet). These larve are therefore compelled to quit their retreats whilst 
searching for their food in a naked state, and they are accordingly 
better fitted for such a kind of life, by having the abdomen of a firmer 
consistence, with stronger anal hooks (jig. 68.18. larva of Hydro- 
psyche atomaria Pictet), and the organs of respiration consisting of 
numerous short bundles (fig. 68. 19. gills of the larva of Hydropsyche 
atomaria Pictet, fig. 68. 20. gills of the larva of Rhyacophila vulgaris 
Pictet), or entirely wanting. Of these species the pupe of the genus 
Hydropsyche are enclosed in a single silky envelope, to which various 
materials are attached; whilst those which have the pupa enclosed, in 
addition to this outer case, in another distinct internal cocoon (jig. 68. 7. 
inner cocoon of Rhyacophila vulgaris Pictet) spun by the larva, com- 
pose the genus Rhyacophila of Pictet. 
In those species which are destitute of external gills, the respiration 
is effected by spiracles placed on each side of each abdominal segment ; 
and species thus circumstanced are placed by Pictet in the same genus 
with others furnished with external organs of respiration. 
From these considerations, M. Pictet has constructed the following 
rs j , avebapods & 
M. V. Audouin communicated to the Entomological Society of France, on the 
9th of January, 1833, a piece of granite, on the surface of which were a number of 
small rounded eminences, formed of minute granules of quartz, and which were dis- 
covered to be the cases of a small species of this family. 
Eee 
a 
