120 A NATURALIST IN BORNEO 



creature is still helpless on the surface. The reason 

 of its inability to sink is found in the simple fact 

 that it is lighter than the water. A full-sized Peri- 

 planeta will weigh more than a small Water-Cockroach, 

 yet the latter can swim in mid-water or even remain 

 quietly at the bottom. If the bodies of these two 

 insects, so similar in structure yet so unlike in their 

 habits, be cut open, a striking difference in the appear- 

 ance of their tracheae is seen. The terrestrial Cock- 

 roach has these breathing-tubes thread-like, silvery, and 

 dilated to their utmost extent with air ; but in the 

 aquatic form they are strap-like, not silvery in appear- 

 ance, and with only an air-bubble here and there to 

 expand them. If any terrestrial insect be examined it 

 will be seen that its tracheae are like those of Periplaneta, 

 and it seems probable that these organs function very 

 largely as storehouses of air, respiration is slower than 

 in aquatic forms, and the tracheae are always distended 

 with air so that the insect is rendered buoyant, and 

 can accomplish with ease movements of running or 

 flying, but on account of their buoyancy they are 

 helpless in water. 1 With the Water-Cockroach and 

 many other aquatic insects the case is very different ; 

 it is essential that they should be able to swim and 

 dive with ease, and this can only be attained if the 

 insects lose their buoyancy ; hence the tracheae must 

 be empty, or nearly so, but as it is necessary that the 

 tissues of the body be constantly aerated, air must be 



1 Insects with strongly developed chitinous exoskeletons are, of 

 course, relatively very heavy, and when thrown into water sink 

 like stones ; but it is astonishing to find how many massively 

 built insects are very buoyant when tested by immersion in 

 water. 



