12 CHARLES T. BRUES. 



very close, even to the difficulty of distinguishing two closely 

 related groups. This is illustrated by the families Chrironomidae 

 and Ceratopogonida 1 , separated in the imagines only by the blood- 

 sucking habits and consequently modified mouthparts of the 

 last family, and not readily characterized in the larval stage by 

 mutually exclusive characters. The same condition is true of 

 two divisions (Tipulidae and Limnobiidae) of the old family 

 Tipulida 1 . In all of these families more difficulty is met with in 

 classifying the pupa?, as we have seen to be the case in the more 

 completely known family of mosquitoes. 



All ot the Diptera so far referred to are not so highly specialized 

 as many other families of the order, although among insects in 

 general the Diptera must be regarded as structurally the most 

 specialized group. Those which we have described, known as 

 the Nematocera, retain in the larval condition some features of 

 the generalized insect, notably in the well developed, free head, 

 with the mandibles moving in opposition in a horizontal plane. 

 To disregard for the moment the families of somewhat inter- 

 mediate structure, and to turn to the higher Diptera, or Cyclor- 

 rhapha, we find larvae of a very different type. They are maggot- 

 like, microcephalic, with the head not differentiated and without 

 appendages, with a simple internal framework, the cephalophy- 

 rangeal skeleton, that bears vertical, hook-like, non-opposable 

 mandibles. Through almost the entire series of families, and 

 in innumerable species, the larvae present almost exactly the 

 same appearance, with only the occasional development of some 

 striking features (e. g., in the genus I'\innia, which has some 

 elaborate tegumentary appendages). In these- larva-, only the 

 posterior pair of spiracles are functional. These, however, are 

 large, and each is provided with three slits. The form, si/e, and 

 position of the spiracles, and the form, size, position, length, 

 curvature and microscopic structure- of the slits present such a 

 complex series of permutations, that these organs in conm-ci ion 

 with certain cuticular structures, usually associated with the 

 spiracular plates, serve for (he identification of very main species. 

 The value of these structures has long been recogni/ed a> valuable 

 in classification and they have been carefully li.uun-d and de- 

 scribed for a large number of species. Certain authors, notably 



