FEEDING AND BREATHING 45 



most spiracles alone open, and these are situated on some 

 outgrowth which can be thrust through the surface film of 

 the water so as to make temporary contact with the upper 

 air. But in other aquatic insect-larvae there is provision 

 for using the air dissolved in the water by means of gills. 

 In the grubs of mayflies and the slender dragon-flies 

 (" damsel-flies ") one or more pairs of abdominal Hmbs — 

 tubular or flattened in form with delicate cuticle and con- 

 taining a network of branching air-tubes — serve for the 

 passage of oxygen from the dissolved air into the tracheal 

 system of the insect. These organs are distinguished as 

 tracheal gills. In the larvae of caddis-flies and many 

 midges there are delicate hollow finger- like outgrowths on 

 the abdomen ; the cavities of these are prolongations of the 

 great blood-space of the body and they are therefore known 

 as blood-gills. In this case the blood-currents pass in and 

 out of these gills, so that the blood serves as a carrier of 

 oxygen to and of carbon dioxide from the tissues, as is usual 

 in animals generally. The well-known '' blood- worm '* 

 larva of the midge Chironomus (Miall and Hammond, 

 1900) has difl"used through its blood-plasma the same 

 respiratory pigment (haemoglobin, a compound containing 

 protein and iron) that is characteristic of vertebrates. These 

 midge-grubs spend much of their time burrowing in mud, 

 and the affinity of the haemoglobin for oxygen, which it holds 

 in loose combination, enables it to retain a store of that 

 *' vital " gas which can be renewed when the slender grubs 

 rise towards the surface of their native pond, where the 

 water is fairly well aerated. 



Some midges of the Chironomus family have, however, 

 larvae without any air- tube system at all, and there are many 

 aquatic grubs in which the special breathing organs become 

 well- developed only late in the process of growth. Such 

 are able to effect gaseous exchanges through the thin and 

 delicate body- wall, as many worms do. Their small size 

 and excessively thin cuticle render any respiratory system 

 needless. The same condition is found throughout life, 

 in most members of the order of wingless insects known as 



