No. 64] DIPTERA OF CONNECTICUT: MORPHOLOGY 45 



Qiesonotum; but the position of the mesothoiaeic, or anteriormost 

 spiracle, sp, indicates that the region immediately in front of it be- 

 longs to the pronotum, ppn of Fig. 6, E. 



The Propleuron. The pronotum is rather closely united with 

 the pleural region in most Diptera (Fig. 6, A and B), and the trans- 

 verse dividing line between the antepronotum. apn, and postpronotum, 

 ^pw., (which corresponds in a general way to the transverse suture of 

 the mesonotum), is continued downward, on each side, in the pleural 

 suture betw^een the episternum, es, and the epimeron, em. 



A small incision separates the prothoracic episternum, fs, from 

 the pronotum above it, in some tipulids (Fig. 6, A), and a rather 

 faint suture is formed in the general region of this incision in a few 

 Diptera, but in most cases the episternum merges with the pronotal 

 region above it. In the lower Diptera (Fig. C, A and B) the protho- 

 racic epimeron, em, is broader than the postpronotal region immedi- 

 ately above it, and is distinct from the episternal region of the meso- 

 thorax behind it. It sometimes forms an arch over the base of th<. 

 coxa, extending from the coxal process, c/, to the mesothoracic epi- 

 sternum. In the higher Di})tera (Fig. 6, C and E) the prothoracic 

 epimeron, em, becomes closely united with the episternal region of 

 the mesothorax, and may merge with the latter more or less com- 

 pletely. 



The coxifer, c/, or ventral process of the pleural region forming 

 the dorsal pivot of the coxa, ex, is formed by a ventral prolonga- 

 tion of the parts of the episternum and epimeron adjacent to the 

 pleural suture (Fig. 6, C and D). In the Avinged hippoboscids (Fig. 

 6, E), the coxifer, <?/, is well developed, and becomes of considerable 

 importance in the movements of the legs of the wingless forms, such 

 as the sheep tick shown in Fig. 6, F. 



The Prostemwn. Four sternal sclerites occur in the prothorax 

 of some orthopteroid insects. The anteriormost sclerite, called the 

 presternum, is probably a detached portion of the basisternum be- 

 hind it. The second sclerite is called the basisternum, or sternum 

 proper, and is frequently connected with the pleural region by a 

 pre-coxal bridge (the occurrence of this bridge is used by Malloch 

 and others in classifying the asilids). The third sclerite, called the 

 furcasternum or "sternellum''', bears the furca, or internal paired 

 apophyses for muscle attachment, and probablj' arose as a demarked 

 posterior region of the plate whose anterior region formed the basi- 

 sternum. The fourth sclerite, called the spinasternum or ''poststernel- 

 luni", bears the spina, or unpaired, internal, median apophysis for 

 muscle attachment. The spinasternum probably arose as a distinct 

 sclerite, although Snodgrass considers that it represents a detached 

 portion of the sternal region behind it. 



The sclerite labelled />/\s- in Fig. 6, C, of the housefly represents 

 the presternum of the lower insects, although this sclerite is absent in 

 most of the lower Diptera. The basisternum, hs, is not connected 

 with the pleural region in the tipulid or the muscid shoAvn in Fig. 

 •>. A and C, but in tlie mycetophilid and culicid shown in Fig, 6, B 



