AQUATIC INSECTS OF FRESHWATER 



FIG. 214. Larva of a 

 Dragon-fly, Gomphus 

 exiijs, with protruded 

 mask. Enlarged. 



Dragon-Flies belong to the order of Odonata, signifying a tooth. 

 More than 2000 species have been established, of which about 300 inhabit 

 the United States. They are very slender insects having four elongated 

 membranous wings, finely 

 netted with veins, each with 

 a jointed structure or nodus 

 near the middle of the 

 front margin. The head is 

 large and may be rotated 

 on the slender neck, and the 

 large eyes are placed at the 

 sides of the head. The an- 

 tennae are short and slender, 

 the mouth parts developed 

 for biting and the legs placed 

 near the front of the thorax 

 and fitted more for grasp- 

 ing their prey and adhering 

 to leaves and twigs than for 

 walking. They are day 



° ' . ^ FIG. Z15. Nymph of a Dragon-fly, 



flyers, the prey being cap- ^m* jumus, with mask folded, not visible 

 . I I , . from above. Enlarged. 



tured by trie legs and man- 



dables in their darting flights. It consists of flies, mosquitoes, midges, 

 gnats and other small insects. All the species are aquatic. The eggs are 

 laid in the water, on the stems or in the tissue of aquatic plants. As soon 

 as the young are hatched they begin their predatory life under water, feed- 

 ing upon eggs, larvas, small insects and entomostraca, which is continued 

 during their entire larval and nymphal existence, attacks being directed 

 to larger insects and animals as they increase in size. The larvas and 

 nymphte. Figs. 214 and 215, have a formidable structure known as a 

 mask, consisting of a long, hinged apparatus with sharp hook teeth which 

 may be folded under the head or darted forward to seize insects and 

 fishes to draw them to the mouth. Most of them breathe by rectal gills, 

 this apparatus also affording the means of locomotion by the violent ex- 

 pulsion of the water. Some have both lateral and caudal abdominal gills. 

 There is a marked difference in appearance between the larva and 

 nymph of most species, the latter assuming a broad, flat form different 

 from the slender larva and the adult insect. Both larva and nymph have 

 short wing pads. When the final metamorphosis takes place the nymph 

 crawls out of the water, the skin splits over the back, and the adult 

 dragon-fly emerges. From nine to twelve months are required to develop 



z6i 



