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APPENDIX B 



are small creatures of similar general aspect, not closely related but grouped here 

 for convenience. 



Class 5. The Centipedes (Chilopoda) are similar in general form to the diplopods, 

 but each segment bears a single pair of legs, the sex openings are posterior, and 

 the anterior pair of legs is modified into poison fangs. 



Class 6. The Insects (Insecta or Hexapoda) are air-breathing arthropods with 

 bodies divided into head, thorax, and abdomen, each body division formed by 

 fusion of segments. The head bears one pair of antennae, the thorax three pairs 

 of- legs. The mouth parts are formed from the appendages of three segments, 

 grouped into a complicated chewing or sucking apparatus. Wings are present on 

 the thorax in most groups. Insects breathe by means of tracheae or tracheal gills. 

 The class is mainly terrestrial, with certain groups secondarily adapted to life 

 in fresh water; marine forms are very few. A high degree of social development 



Fig. B.21. Representative lower insects. A and B are primitive wingless insects, Apterygota; 

 all the rest belong to the winged group, Pterygota, though some have lost wings. A, Lepisma, 

 a bristletail or silver fish. B, a collembolan or springtail. C, a grasshopper, Orthoptera. D, 

 an earwig, Dermaptera. E, a body louse, Anoplura. F, a mayfly, Ephemeroptera. G, a 

 winged (sexual caste) termite or white ant, Isoptera. H, a book louse or bark louse, Corro- 

 dentia. /, a biting louse or bird louse, Mallophaga. J, a giant water bug or electric-light 

 bug, Hemiptera. K, a cicada or harvest locust, Homoptera (often treated as a suborder of 

 Hemiptera.) L, a dragonfly, Odonata. (Modified from Turtox chart, courtesy General Biologi- 

 cal Supply House, Inc.) 



has been reached by the termites, ants, bees, and wasps. Insects are more numer- 

 ous in species and probably in individuals than all other animals taken together; 

 something over 600,000 species are said to have been described, and thousands 

 of previously unknown species are named each year. Many of man's most serious 

 competitors and enemies are insects. The essential roles of some of the flower- 

 visiting insects in pollination are well-known, and members of this group form the 

 basic food supply for numberless food chains. The principal subclasses and orders 

 of insects are as follows: 



Subclass 1. Apterygota, primitively wingless insects with little or no meta- 

 morphosis. Includes the bristletails (Thysanura) and springtails (Collembola). 

 Members of the latter group are the oldest known insects, fossils having been 

 found associated with psilophytes in Devonian rocks. 



Subclass 2. Pterygota, the winged insects. Wings usually present, sometimes 

 reduced or lost; no abdominal appendages except at tip. Among the winged 

 insects two grades or divisions may be recognized — a lower and a higher — dis- 

 tinguished chiefly by their mode of development. 



