224 THE ZOOLOGIST. 



This same feature may be observed in many terrestrial Blattidce, 

 so that it cannot be regarded as associated with the aquatic habit. 

 A microscopic examination of one of these spiracular tubes reveals 

 the following features (fig. 3). The orifice of the tube leads into 

 a short vestibule (v.), the vestibule joins a large trachea (*.)> but 

 intervening between vestibule and trachea is the spiracle (s.)> a 

 narrow slit in a diaphragm ; the slit is opened and closed by the 

 action of a chitinous bow (&.), worked by a muscle (m.) attached 

 to the wall of the vestibule. This is essentially the structure of 

 all the abdominal spiracles in Cockroaches, and the terminal 

 spiracular tubes of the aquatic species are merely enlarged 

 equivalents, shifted dorsally, of the short spiracular plates of the 

 preceding segments (fig. 2, s.), which are situated on the ventral 

 side of the abdomen. 



Externally, at any rate, the aquatic Cockroaches exhibit no 

 particular modifications for their remarkable habit of life, the 

 legs are not different from those of allied terrestrial genera, and 

 there is nothing in their general appearance to suggest their 

 aquatic habit of life. During the experiments that have been 

 described one distinctive feature, however, in the economy of the 

 insects was notable, viz. the ease with which they could remain 

 below the surface of the water. Most adult aquatic insects, e. g. 

 Dytiscus, Corixa, Notonecta, can only keep below the surface by 

 continuing to swim, or by propping themselves under some stone 

 or submerged leaf ; directly they relax their efforts they float to 

 the surface. Again, if a large heavy Cockroach, such as Panesthia 

 javanica, is thrown into water, it flounders helplessly on the 

 surface, and is quite unable to sink ; whereas the much lighter 

 aquatic Cockroach is able to swim, to dive, and to remain sub- 

 merged with great ease. An explanation of these facts is found 

 if the tracheal systems of the insects are examined. The tracheae 

 of Dytiscus and of Panesthia present the usual appearance of 

 opaque silvery tubes filled with air ; the tracheae of the aquatic 

 Cockroaches, on the other hand, are transparent, flattened, strap- 

 like structures, dilated here and there only with air-bubbles. 

 Dytiscus and Panesthia are buoyed up in water by the plentiful 

 supply of air stored in their bodies, but the tracheae of the water 

 Cockroach are mere air-passages, not storehouses, the respiratory 

 movements are rapid, causing a constant circulation of air, and 



