262 NATURAL HISTORY OF AQUATIC INSECTS CH. 



had broken down the perforated ends of their sheaths, 

 and had partly emerged. He has also seen such 

 pupae transformed into flies. I have had flies which 

 emerged in the beginning of April, having probably 

 passed the winter as pupae in their sheaths. 



" Sometimes the sheath of the larva is longer than 

 necessary for it in the pupal stage. In such cases 

 perforated partitions are formed across the tube at 

 two points, but if the larval case is short, the per- 

 forated plates are fixed at the two extremities. I 

 have seen at times the perforated ends of the pupal 

 sheaths move in and out according as the Insect 

 sucked the water in or expelled it. These pupal 

 sheaths are generally attached to some fixed object, 

 and the sheath is fixed before the perforated plates 

 are added." 



The pupal case is often a good deal shorter than 

 that in which the larva dwelt. Before pupation the 

 larva makes provision for its security during the rest- 

 ing stage. Various plans are employed by different 

 species. Some close the ends of the tube with the 

 silken sieves described by Reaumur ; others (Limno- 

 philus auricula, L. griscus, Halesus) with stones ; 

 Odontocerum with a single stone. Micropterna 

 sequax is said by Pictet l to make its case at first of 

 leaves ; as the larva grows it adds stones, until at 

 last the case is entirely stony. When pupation ap- 

 proaches, the case is lengthened with larger stones, 

 and closed at the wide end. Then the larva proceeds 

 to bury the narrow end vertically in the mud, until only 

 the large stones at the wide end are visible. To accom- 

 1 Recherches sur les Phryganides, p. 133. 



