24 Till: Ur'K-STOHY OF INSECTS [CH. 



they pass entirely in the air. The preparatory in- 

 stars of such insects are aquatic ; the adult instal- 

 ls aerial. All may-flies, dragon-flies, and caddis- 

 flies, many beetles and two-winged flies, and a few 

 moths thus divide their life-story between the water 

 and the air. For the present we confine attention 

 to the Stone-flies, the May-flies, and the Dragon-flies, 

 three well-known orders of insects respectively called 

 by systematists the Plecoptera, the Ephemeroptera 

 and the Odonata. 



In the case of many insects that have aquatic 

 larvae, the latter are provided with some arrange- 

 ment for enabling them to reach atmospheric air 

 through the surface-film of the water. But the larva 

 of a stone-fly, a dragon-fly, or a may-fly is adapted 

 more completely than these for aquatic life ; it can, 

 by means of gills of some kind, breathe the air dis- 

 solved in water. 



The aquatic young of a stone-fly does not differ 

 sufficiently in form from its parent to warrant us in 

 calling it a larva; the life-history is like that of a 

 cockroach, all the instars however except the final 

 one the winged adult or imago live in the water. 

 The young of one of our large species, a Perla for 

 example, has well-chitinised cuticle, broad head, 

 powerful legs, long feelers and cerci like those of 

 the imago ; its wings arise from external rudiments, 

 which are conspicuous in the later aquatic stages. 



