4 AQUATIC COCKROACHES. 
This same feature may be observed in many terrestrial Blattide, 
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. 8). 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 (b.), 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, Coriza, 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 trachez 
of Dytiscus and of Panesthia present the usual appearance of 
opaque silvery tubes filled with air; the trachee 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 trachez of the water 
Cockroach are mere air-passages, not storehouses ; the respiratory 
movements are rapid, causing a constant circulation of air, and 
