14 Bulletin American Museum of Natural History [Vol. XLII 



The spiracular area, which bears the spiracle, is just lateral to the 

 tergum. 



The pleural region is composed of the hypopleurite and the epi- 

 pleurite, the latter above the former. They are usually fairly distinct 

 and bear lobes. The sternal region usually consists of the same number 

 of folds as the tergal region but is much more complex. As in the latter, 

 there are corresponding folds known as the presternum, sterna (two 

 folds) and sternellum. 



The stigmatic atrium, or caudal breathing apparatus, offers oppor- 

 tunity for controversy as to the segments which compose it, but I have 

 considered it as formed by the modified eighth and ninth segments. 

 The procercus is the process of the eighth pleurite; the mesocercus, or 

 true cercus, is always an appendage of the ninth segment ; the acrocercus 

 is an appendage of the lateral lobe of the ninth segment and arises from 

 its ventral side; and the prostyle ("flotteurs" of Portier), which pre- 

 cedes the anus, is an appendage of the tenth segment. The motory 

 stylus is a fleshy process of the pupa, resembling a seta. It may or may 

 not bear a terminal seta and is usually annulate. The pterotheca is the 

 pupal covering of the adult wing. 



General Characters of the Family (Immature Stages) 



The eggs are whitish (brownish in Hydroscapha) , oval in shape, and 

 with a small projection at the anterior end. The eggs, varying in 

 number from one to one hundred and forty odd, are usually enclosed in a 

 silken case 1 but, in the more primitive forms (Ochthebius, Limnebius, 

 and Hydroscapha), the eggs are deposited singly, without any covering 

 or with one of loosely applied silk. Cymbiodyta and Paracymus lay their 

 eggs in masses but cover them only sparsely with silk. The cases are 

 either free floating (Hydrous, Hydrophilus) ; attached to water plants or 

 debris at the surface of the water (Tropisternus, Philydrus, Hydrobius, 

 Laccobius, Anccena); below water (Berosus); embedded in mud, moss, 

 etc. (Hydrobius, Helophorus); carried on the under side of the body 

 (Spercheus, Epimetopus, Helochares, Chcetarthria f) : or placed in dung or 

 damp earth. The egg-masses of Cymbiodyta and Paracymus and the 

 single eggs of the more primitive species are deposited under fallen leaves, 

 vegetation or stones near the edge of the water. 



iStein, 1847, who found silk glands in many genera, supposed that all eggs were covered with silk 

 but the degree of covering varies greatly. 



