MANUAL OF THE APIAET. 59 



titute of these discs, it is almost wholly fluid, and is almost 

 wholly made up of nutritious substance. 



The respiratory or breathing system of insects has already 

 been referred to. Along the sides of the body are the spira- 

 cles or breathing mouths, which vary in number. These are 

 armed with a complex valvular arrangement which excludes 

 dust or other noxious particles. These spiracles are lined 

 with a delicate membrane which abounds with nerves, which 

 were referred to in speaking of them as smelling organs. 

 From these extend the labyrinth of air- tubes (Fig. 2, f,f'), 

 which breathe vitalizing oxygen into every part of the insect 

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

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

 large air-sacks (Fig. 2, f). Insects often show a respiratory 

 motion, which in bees is often very marked. Newport has 

 shown that in bees the rapidity of the respiration 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 mercury in a thermometer 

 which he suspended in the hive amidst the cluster. In the 

 larva 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 rectum, or last part 

 of the intestine. This insect, by a muscular effort, draws the 

 water slowly in at the anus, when 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 a mode of motion. In the pupae of insects there is little 

 or no motion, yet important organic changes are taking place — 

 the worm-like, ignoble, 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 and pollen. The 

 pupae no less than the larvae of some two-wing flies, which 

 live in water, have long tubes which reach far out for the 



