SURROUNDINGS OF GROWING INSECTS 191 



segments in the form of thread-like or finger-like " blood- 

 gills " (g). Through the thin walls of these delicate outgrowths 

 of the grub's body gaseous exchange goes on between the 

 contained blood and the air dissolved in the surrounding water, 

 oxygen being absorbed and the products of combustion 

 eliminated. Many Chironomus larvae are bright red in colour, 

 being known as " blood- worms ". They form tubular shelters 

 at the bottom of the ponds and ditches which they inhabit, 

 by fastening together small fragments of soil. The grub may 

 often be seen thrusting its hinder end out of this protective 



FIG. IOO. 



a. Harlequin Midge (Chironomous) , male ; 6, larva (g, gill-filaments) ; c, pupa (tr, tracheal 

 gills), x 4. After Johannsen, Bull. 86, New York State Museum. 



sheath and waving its delicate gill-processes vigorously in the 

 water, so as to facilitate the absorption of the dissolved oxygen. 

 Even while remaining within the tube it practises a wave-like 

 movement of the body, which keeps fresh water-currents 

 constantly bathing it. This action suffices for ordinary needs, 

 but sometimes a richer supply of oxygen becomes a necessity ; 

 then it leaves its tube altogether, rising to the surface region 

 of its native pond or ditch, where the water is less poorly 

 aerated than in the depths. The red colouring-matter of 

 these " blood worms " is similar to the haemoglobin of our 



