THE INSECTS: HEXAPODA 



97 



species, belong to the family of long-horn or true grass- 

 hoppers, Locust' idee. The crickets (GrryVlidce) and the cock- 

 roaches (Blat'tidce) are allied families. By common consent 

 family names end in -idee. Families are united to form orders, 

 and orders in turn make up classes. The largest and most 

 important of the orders which make 

 up the class Hexapoda have already 

 been discussed. As we shall see 

 later, the classes are united to form 

 phyla (sing., phylum), the primary 

 divisions of the animal kingdom. 



Definition of Thysanura (Gr. 

 thysanos, fringe ; our a, tail). Of 

 the orders not mentioned in the 

 preceding chapters it will be neces- 

 sary to refer only to the Thysanu'ra, 

 or springtails. The springtails are 

 small, flattened, wingless insects, 

 with usually simple eyes, and ap- 

 pendages on some of the abdom- 

 inal somites. They are found under 

 stones, in damp places, or in human 

 dwellings. They develop from the 

 egg without metamorphosis. The 

 best-known species is the " silver- 

 fish," or " fish, moth " (Lepis'ma sac- 

 chari'na, Fig. 54), often seen in houses, where it sometimes 

 does damage to starched clothing or to the bindings of books. 



Generalized and Specialized Forms. The Thysanura are espe- 

 cially interesting, since they probably more closely resemble 

 the ancestral type of insect than do any other living species. 

 The primitive or ancestral forms were probably naked, wing- 

 less hexapods, without strongly marked divisions into head, 

 thorax, and abdomen. Their legs were probably equal or 



FIG. 54. Fishmoth. Enlarged. 

 (After Marlatt, Bulletin No. 4, 

 N.S., United States Depart- 

 ment of Agriculture, Division 

 of Entomology) 



