HEMIPTERA. 143 



from biting mouth parts. This form of the larva, 

 therefore, probably arose from Thysamiroid-hke an- 

 cestors having the usual form of mouth parts, bat the 

 excessive acceleration of development in forms of 

 later occurrence has brought about the loss of all the 

 transitional characteristics, and all traces of this origin 

 are now skipped or left out of the stages of growth 

 in the development of the mouth parts of existing 

 species. 



There are several groups of the Heteroptera in 

 which the development is more or less accelerated 

 in other respects, the Thysanuriform stage of larval 

 growth being abbreviated or perhaps absent, so far as 

 relates to the equal proportions of the thoracic rings. 

 The larvae, in other words, Hke those of the more 

 specialized groups of Orthoptera, mentioned above, 

 resemble their own adults, the ancestral Thysanuriform 

 stage being either wholly or in large part skipped. 



The larval forms and adults of Homoptera present 

 greater departures from the generalized type of Thy- 

 sanura than those of the normal Heteroptera. The 

 curious similarity between the wings and halteres of 

 the adult males of some of the Coccidae (scale in- 

 sects) and the same organs of Diptera is a notable 

 example of this divergence, and is also one of the 

 most remarkable examples among insects of the in- 

 dependent origin of similar characteristics in different 

 orders, as has been stated above. 



The extraordinary larvae of the Cicadas, which Dr. 

 Packard regards as the most highly specialized of the 

 Hemiptera, are certainly in many respects very wide 

 departures from the Thysanuroid standard. They are 



