38 GUIDE TO INSECTS. 



Table- grown it makes its way to the surface of the water, the skin splits 

 case ' and the winged insect emerges. This process occupies a very short 

 time, sometimes only a few seconds. This winged form, called the 

 sub-imago, is, however, still enveloped in a delicate skin ; this it 

 throws off either immediately or soon, and the insect is then in its 

 perfect state. 



The food of the larvae is chiefly vegetable matter, but some 

 species are at least in part carnivorous. 



Some species of May-fly occur in swarms and appear in the air 

 like a fall of snow. Specimens swept from a railway platform in 

 Egypt after one of these swarms had occurred are exhibited (1409). 

 A few of another swarm from Germany are in the same case in 

 spirit (1403). 



Sub-Order ODONATA. 



These insects, popularly called Dragonflies (1410-1453), are 

 insects with incomplete metamorphosis. The head is very large, 

 concave behind, with very slender attachment to the thorax, so that 

 it has complete freedom of action. The eyes are very large, some- 

 times touching each other above. The antennae are very short and 

 terminate in a bristle. The wings are equal or very nearly so. 

 Although transparent they are somewhat hard and brittle. The 

 veins form a network. The abdomen is very long. 



One great peculiarity of this Sub-Order is the form of the thorax. 



Fig. 39. 

 a. 



c 



Side view of the thorax of Mecistogaster. 

 a, prothorax ; &, mesothorax ; c, metathorax ; d, d, bases of wings. 



When viewed sideways the segments are seen to slant, so that the 

 legs are in front of the wings (fig. 39). In other insects the base 

 of the legs is under the base of the wings. The prothorax is very 



