GENERAL ZOOLOGY 



Fig. 15.25. Protura. A typical proturan, Acerenlulus barben. Note 

 the simple eyes and the absence of antennae. (Redrawn, after 

 H. E. Ewing, from H. H. Ross, A Textbook of Entomology, second 

 edition, copyright 1956 by John Wiley and Sons, Inc., printed 

 by permission.) 



Order Thysanura (tassel tail) — generalized, wingless insects, some species 

 of which possess such primitive characteristics as vestigial abdominal ap- 

 pendages and paired external genital openings. Common examples are the 

 household pests known as silverfish, and the firebrat, Thermobia (Fig. 15.26). 

 The mouth parts are mandibulate, and the life cycle is ametabolous. 



Order Isoptera (equal wings) — termites. These primitive insects are in- 

 teresting from several biological standpoints, as well as from the economic 

 point of view related to their wood-eating habit and the consequent damage 

 to houses and other wooden structures. The termites are primarily tropical 

 forms but occur in warm temperature regions also. We have mentioned 

 (p. 243) the array of hypermastigote flagellates which inhabit the intestinal 

 tracts of termites and other wood-eating insects; this appears to be a mutual 

 relationship, the insect depending on the capacity of the protozoans to digest 

 cellulose. Although termites are structurally relatively simple insects, they 

 have developed a social structure which to some extent parallels that of the 

 social Hymenoptera (pp. 470-475). The social unit, or colony, is organized 

 about a pair of functional reproductive individuals, the king and queen, 

 responsible for the production of all the zygotes that develop into members 

 of the society. In addition, there are variable numbers of sterile soldiers, 

 or both sterile soldiers and sterile workers, and larger numbers of nymphs. 

 Each of these castes exhibits morphological and behavioral specializations 

 related to the maintenance of the colony. Experimentation has demonstrated 

 that the numbers in each caste are maintained in constant ratio, and that 

 the factors responsible for this coordination include specific substances which 

 are transmitted between individuals. Brieflv, some of the evidence is as 



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