SURROUNDINGS OF GROWING INSECTS 197 



through the water by undulating movements of its abdomen, 

 often in constant motion diving to the bottom of its native 

 ditch or rising to the surface. When it rises it can bring 

 together the points of the processes of its spiracular siphon, 

 and it is thus capable of piercing the surface-film (Fig. 104 a}. 

 Then, separating these points, it exerts a pull on the tensile 

 surface so that a little cup-like depression is formed from which 

 the siphon and indeed the whole larva is, as it were, suspended, 

 so that its air-tube system is open to the atmosphere ; it takes 

 in air through the tail-spiracles directed upwards while it can 

 feed through the downwardly-directed mouth. After thus 

 hanging from the surface-film for a time, the larva closes the 

 cup by bringing together the points of its processes and releases 

 itself ; then with a supply of fresh air in its tracheal system, 

 it dives and moves again with vigorous graceful flexions of its 

 abdomen through the deeper layers of the ditch-water where 

 food is more abundant than near the surface. 



Among the details already given of the aquatic larva and 

 pupa of the midge Chironomus it will be remembered that the 

 gills of the larva are at the hinder end of the abdomen while 

 those of the pupa are on the prothorax. A closely similar 

 change in the position of the breathing organs is seen in the 

 transformations of Culex. The pupa (Fig. 102 d) of the gnat, 

 with its relatively very large, rounded thorax and its elongate, 

 freely- jointed abdomen with terminal fin-like processes (e), is 

 capable of a considerable degree of movement, and is far less 

 passive in habit than are most insectan pupae. Like the larva 

 it breathes atmospheric air, but unlike the larva its spiracles 

 (sp) are on the prothorax at the end of a pair of elongate 

 tubular or trumpet-shaped outgrowths, so that the pupa hangs 

 from the surface film with the head and tail ends both point- 

 ing downwards and the dorsal thoracic region uppermost. 

 This arrangement has drawn admiring comment from students 

 of insect transformations both in early and modern days. 

 For this dorsal thoracic region, close to the surface, is the 

 portion of the pupal cuticle that splits open to allow the emer- 

 gence of the gnat, and the elongate feelers, jaws, legs, wings, and 

 abdomen of the imago have to be withdrawn from the pupal 

 structures wherein they have developed towards this region 

 so as to be extricated from the cast pupal envelope. And the 



