AQUATIC INSECTS 88 1 



The Odonata are distinguished by the venation of the wings, 

 especially by the possession of a distinct nodus and stigma of the 

 type shown in Fig. 1388. 



The Ephemerida are distinguished by the venation of the wings 

 (Fig. 1387), and by their proportions and their extensive corruga- 

 tion. 



The Hemiptera are distinguished by the possession of a jointed, 

 sucking proboscis, directed backward beneath the head and 

 thorax. 



The Trichoptera are distinguished by the hairy covering of 

 their wings, the absence of jaws and proboscis (palpi are pres- 

 ent) and by a type of venation of wings similar to that shown 

 in Fig. 1391. 



The Lepidoptera are distinguished by their covering of powdery 

 scales, and by the possession of a coiled sucking proboscis. 



The Coleoptera are distinguished by the hardened fore wings 

 (elytra) meeting in a straight line down the middle of the back. 



The Diptera are distinguished by the possession of a single pair 

 of wings, with very few cross- veins in them (Fig. 1378). 



The other two orders, Plecoptera and Neuroptera, lack the above 

 combinations of characters and may be readily recognized by their 

 general likeness to figures published in the following paragraphs 

 devoted to them. 



Besides these nine orders, there are three others, of slight impor- 

 tance in the life of the water, that are deliberately ignored. These 

 are: 



(1) The Thysanura, or springtails, common on the surface of 

 water, but not living in it. They will be readily recognizable, if 

 collected, by their very minute size, entire absence of wings, mouth 

 parts retracted within the head, and the forked spring beneath 

 the abdomen by means of which they jump freely. 



(2) The Orthoptera, of which some of the grouse locusts (family 

 Tettigidae) , living by the water side, occasionally jump in and take 

 a swim. 



(3) The Hymenoptera, of which a few minute egg parasites, 

 enter the water as adults to find the eggs of their aquatic victims, 

 and these swim with their wings (Polynema, etc.). 



