THYSANURA 



223 



The ventral sacs are sac-like expansions of the wall of the coxites 

 which can be everted, probably by blood -pressure, and are withdrawn 

 into the cavity of the coxite by muscles (Fig. 225). In Figure 221, 

 the openings into the retracted ventral sacs are represented ; there is 

 one pair in the first abdominal seg- 

 ment; two pairs in each of the four 

 following segments:; and a single pair 

 each in the seventh and eighth ab- 

 dominal segments. In Lepisma the 

 ventral sacs are wanting. The func- 

 tion of the ventral sacs has not been 

 definitely determined; but it seems 

 probable that they are blood-gills. 

 The presence in the Thysanura of 

 styli and of ventral sacs, which are 

 evidently homologous with those of 

 the S3anphyla, is an indication of 

 the primitive condition of these 

 insects. The generalized form of 

 the reproductive organs of the Thy- 

 Fig. 226. — sanura is another indication of this. 

 Ovary of Ja- In Japyx the ovarian tubes have a 

 p/x. (After metameric arrangement (Fig. 226); 

 rassi.; ^^^ -^^ Machilis (Fig. 227) we find an 



intermediate form between a metameric arrange- 

 ment of the ovarian tubes and a compact ovary. 

 These facts, and especially the presence of styli and 

 ventral sacs, are opposed to the view held by some 

 writers that the Thysanura are degenerate instead 

 of primitive insects. It is true that degenerate fea- 

 tures are present in the order, as the loss of eyes in 

 Japyx and Canipodea; but this loss is correlated with 

 the life of these insects in dark places, like the loss of eyes in certain 

 cave-beetles, and is not important in the determination of the 

 zoological position of the order. 



The young of the Thysanura resemble the adults in form, there 

 being no marked metamorphosis. In Campodea and Japyx the molt 

 is partial (Grassi '89). 



This is a small order; less than twenty American species have 

 been described ; these represent four families. The distinguishing 

 characteristics of the families are given in the following table. 



A Body clothed with scales. With three filiform caudal appendages, a pair of 

 cerci and a median caudal filament. Compound eyes present. Jaws not over- 

 grown by folds of the gense. (The Ectotrophi). 



B. The abdominal tergites reflexed to the under surface so as to form an 

 imbrication covering the sides of the coxites (Fig. 221). Compound eyes 

 large and contiguous. Prothorax smaller than the mesothorax. Middle and 



hind legs with styli. Saltatorial insects Machilid^ 



BB. Abdominal tergites not covering the sides of the coxites. Eyes small and 

 distant. Prothorax as large as or larger than the mesothorax. Middle and 

 hind legs without styli. Not saltatorial insects Lepismatid^e 



Fig.227. — Ovary 

 of Machilis: c, 

 coxite of the 

 eighth abdomi- 

 nal segment; s, 

 stylus ; o, ovi- 

 positor. (After 

 Oudemans.) 



