24 ENTOMOLOGY 



that is surprising in view of the many strong differences of structure 

 between the two groups, particularly as regards the mouth parts. 

 Though investigators have not agreed as to the morphology of the 

 mouth parts of the sucking lice, a study of cross sections of the mouth 

 parts leads to the conclusion that they conform fundamentally to the 

 hemipteran type. 



Metamorphosis offers the broadest criteria for the separation of 

 insects into primary groups. All the orders considered thus far are 

 characterized either by no metamorphosis or by a relatively slight, or 

 so-called direct, or incomplete, transformation. The following orders, 

 on the contrary, are distinguished by an indirect, or complete, metamor- 

 phosis, which appears in Coleoptera and attains its maximum develop- 

 ment in Hymenoptera and Diptera. 



With Coleoptera the eruciform type of larva appears, as a derivative 

 of the earlier thysanuriform type. The larvae of Meloe, Epicauta (Fig. 

 220) and other genera pass from a thysanuriform stage to an eruciform 

 condition during their development. 



It was formerly thought that the resemblances between Coleoptera 

 and Dermaptera were superficial, but at present there is reason to 

 believe that these two orders are related. They agree rather closely 

 in structure, especially as regards the structure of the head (Cramp ton) 

 and the thoracic sclerites (Snodgrass). Coleoptera have affinities 

 with Neuroptera also, that appear in some of the larvae as well as in the 

 adult forms. Coleoptera are, however, more primitive than Neurop- 

 tera, and are placed here at the beginning of the holometabolous series. 



Strepsiptera should be separated from Coleoptera as a distinct 

 order, accepting the opinion of Dr. W. D. Pierce, who has studied the 

 group thoroughly. Strepsiptera are aberrant, peculiarly specialized 

 forms. The fact that the male strepsipteran pupa has the form of a 

 hymenopterous pupa may or may not be significant. 



In Neuroptera, as in Coleoptera, the transition from the thysanu- 

 riform to the eruciform type of larva may take place during the develop- 

 ment of the individual, as in the larva of Mantis pa. 



Neuroptera have kinship with Coleoptera; the structure of the head, 

 for one thing, being essentially the same in the two groups. They 

 resemble Plecoptera also; thus a form like Sialis may have come from 

 ancestors like perlids. 



All the orders that follow are derived from the neuropteran stem, 

 in the opinion of many authorities. 



Mecoptera form an isolated order, though their caterpillar-like 



