70 



CONXECTICUT GEOL. AXD XAT. HIST. SURVEY [Bull. 



The fact that the anal opening is always dorsal in relation to tlie 

 venti-all}' located genital openings of lower insects is of some value 

 for determining Avhich structures are tergal and Avhicli are sternal 

 when an inversion or similar displacement of the parts occurs in the 

 Diptera. Furthermore, the cerci occurring on each side of the anal 

 opening in the higher Diptera, represent the appendages of the 

 eleventh abdominal segment of lower insects, so that it is extremely 

 probable that the eleventh abdondnal segment enters into the com- 

 position of the composite, cerci-bearing, so-called "anal segment" 

 occurring behind the ninth or genital segment of the males of the 

 liigher Diptera. 



2. Morphology of the abdomen 



The segments of the abdomen are called uromeres or urites, and 

 the dorsal plates of the uromeres are usually called tergites,* while the 

 ventral plates are called sternites, and the lateral plates, wdien pres- 

 ent, are called pleurites. The membranes betw^een the tergites and 

 sternites are the lateral or pleural membranes, while the membranes 

 between the consecutive segments are the intersegmental membranes, 

 or conjunctivae. 



The extent of the pleural membrane varies greatly even among 

 tlie mem.bers of the same group of Diptera. Thus, in the calobatid 

 Calohata (Fig. 10, F), the tergites and sternites are Avidely separated 

 by the extensive pleural membrane, while in certain other Cyclor- 

 rhapha, such as the tachinid Thelaira^ the pleural membrane is re- 

 duced to the narrow inflexed areas between the greatly enlarged ter- 

 gites and the greatly reduced sternites, which are overgrown by the 

 enlarged tergites. In some phorids, and in such chloropids as CMo- 

 rops, etc., the entire sternal region may become membranous, while 

 in some orphnephilids, hippoboscids, etc., the greater portion of the 

 abdomen itself may become membranous, and in such cases the 

 segmentation is largely indicated by the position of the spiracles. 



The Spiracles. Originally, the first eight abdominal segments 

 bore spiracles, and eight pairs of spiracles occur in some tipulids, 

 bibionids, rhagionids (leptids), mydaids, etc., while seven or eight 

 pairs are found in certain chironomids, orphnephilids, stratiomyids, 

 scenopinids and asilids; the occurrence of seven pairs of spiracles 

 is rather typical of the Diptera m general, and male Diptera appar- 

 ently never have more than this number. Some chloropids and ephy- 

 drids have only six pairs of spiracles, and some anthomyids have 

 only five pairs, although the reduced number is of comparatively rare 

 occurrence. 



The spiracles are typically borne near the middle of the segment, 

 although the first pair usually occurs in the anterior region of the 

 basal segment (or in the posterior region in such forms as Oncodes), 

 whde in so me cases {Allognostci) all of the spiracles tend to shift 



♦Since recent morphologists apply the designations tergites, sternites. etc, to sub- 

 divisions of the terga, sterna etc, it is suggested that the terga sterna etc of the 

 abdomen might be referred t6 as uroterga, urosterna etc steina, etc., ot the 



