AQUATIC INSECTS 



and adult stonefly (Fig. 144). The mouth parts are a labrum 

 or upper lip, labium or lower lip, and two pairs of jaws, the 

 mandibles and maxillse, which open and close sidewise. In 

 the stonefly nymph the labrum is a simple flap hanging for- 

 ward over the upper jaws (Fig. 139). 



Fig. 139. — Mouth-parts of a stonefly nymph seen 

 from beneath: A. — With labium (i) in place; 2, 

 maxilla; 3, labrum. B. — With labium removed; i, 

 maxilla; 2, mandible. 



The labium folds upward and covers the maxillae or lower 

 jaws; it is partly slit through the middle and bears a three- 

 jointed feeler-like palp upon each side. The upper jaws 

 or mandibles are single pieces of chitin, saw-toothed at the 

 tips for biting and chewing. The maxillae are also used in 

 chewing but are more delicate and each is made up of several 

 parts including a five- jointed palp. Insects test their food 

 with their palps before they take it into their mouths, the 

 hypopharynx or tongue being strongly coated with chitin 

 and of little use as a sense organ. 



Three pairs of legs on the thorax distinguish insects from 

 all other arthropods, spiders and their kin and the crustaceans; 

 one or two pairs of wings are also borne there. The thorax 

 is divided into three parts, prothorax, mesothorax, and meta- 

 thorax, on each of which is one pair of legs. The first pair 

 of wings is on the mesothorax and the second pair on the 

 metathorax. Developing wings or wing-pads of the stone- 

 fly nymph can be seen on its thorax (Fig. 139). These are 



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