Respiratory System. 65 



each side of the abdomen. These are armed with a com- 

 plex valvular arrangement which excludes dust or other 

 noxious particles. From these extends the labyrinth of 

 air-tubes (Fig. 2,_/,y,), which carries vitalizing oxygen 

 into every part of the insect organism. As shown long 

 ago by Leydig and Weismann, these air-tubes are but an 

 invagination of the derm of the insect. What is more 

 curious, these tracheae are molted or shed with the skin of 

 the larvae. In the more active insects — as in bees — the main 

 tracheae, one on each side of the abdomen, are expanded 

 into large air-sacs (Fig. 2,_/"). Insects often show a 

 respiratory motion, which in bees is often very marked. 

 Newport has shown that in bees the rapidity of the respira- 

 tion, which varies from twenty to sixty per minute, gauges 

 the heat in the hive, and thus we see why bees in times of 

 severe cold, which they essay to keep at bay by forced 

 respiration, consume much food, exhale much foul air and 

 moisture, and are liable to disease. Newport found that 

 in cases of severe cold there would be quite a rise of mer- 

 cury in a thermometer which he suspended in the hive 

 amidst the cluster. 



In the larval state, many insects breathe by fringe-like 

 gills. The larval mosquito has gills in form of hairy tufts, 

 while in the larval dragon-fly the gills are inside the rec- 

 tum or last part of the intestine. The insect, by a muscu- 

 lar effort, draws the water slowly in at the anus, where it 

 bathes these singularly placed branchiae, and then makes it 

 serve a further turn by forcibly expelling it, when the 

 insect is sent darting ahead. Thus this curious apparatus 

 not only furnishes oxygen, but also aids in locomotion. In 

 the pupae of insects there is little or no motion, yet import- 

 ant organic changes are taking place — the worm-like, igno- 

 ble, creeping, often repulsive larva, is soon to appear as the 

 airy, beautiful, active, almost ethereal imago. So oxygen, 

 the most essential — the sine qua non — of all animal food is 

 still needed. The bees are too wise to seal the brood-cell 

 with impervious wax, but rather add the porous capping, 

 made of wax from old comb and pollen. The pupae no 

 less than the larvae of some two-winged flies which live in 

 water, have long tubes which reach far out for the vivify- 



