186 Annals Entomological Society of America  [Vol. XIV, 
position of the postabdomen the right lateral conjunctiva often makes 
a loop cephalad under the terminal sternite of preabdomen, following 
the segments introverted into the genital pouch. The right spiracle of 
urite six, when this segment is transformed into postabdomen retains 
its nearly normal position near the caudo-lateral corner of the preceding 
untransformed segment (Figs. L, M); that of urite seven may lie near 
the right side of the preabdomen or, together with the eighth, may be 
displaced far ventrad and often senestrad (Fig. L) to well beyond the 
mid-ventral line of the preabdomen. 
The lateral conjunctiva of the left side is much less distorted (Fig. N). 
It bends dorsad and slightly toward the right beyond the apex of the 
preabdomen, following the spiral twist of this part of the abdomen. 
The left spiracle of urite six usually lies very close to the caudo-senestral 
corner of the fifth sternite, though it may be withdrawn underneath 
(dorsad of) the latter into the genital pouch. The left spiracles of 
urites seven and eight are displaced dorsad and also often caudad to near 
mid-length of their respective segments (Figs. K, L). They are not 
much displaced toward the right, so that both spiracles of urites seven 
and eight often lie to the left of the mid-ventral line of the preabdomen. 
The true relationship of the sclerites in the postabdomen is much 
obscured in many genera by three specializations. (1) The left lateral 
conjunctiva has practically disappeared in some cases by the fusion of 
sternites seven and eight to tergites seven and eight respectively, on 
their left side (Fig. K); thus making practically a single sclerite of each 
of these segments with an evident break only at the right conjunctiva. 
However the spiracles clearly remain on the left side to show the 
homologies. In Mesogramma geminata for example (Fig. K, Plate IX), 
the break between sternite seven and tergite seven on the left side is 
indicated only by a slight change in density of chitin and a trace of a 
suture in front and behind, while on the eighth urite all that remains 
of the lateral conjunctiva is a trace of the suture over the cephalic half 
leading back to the spiracle. 
(2) The second obscuring tendency is toward the disappearance 
of the tergites of six, seven and eight or rather, their dechitinization. 
It is as though the pressure from the genital appendages prevented in 
some way the laying down of chitin in those parts of the segments that 
line the genital pouch. 
The tergites of six, seven and eight are very well chitinized in at 
least a part of the species of the following genera: Chilosia, Meso- 
gramma, Sphegina, Baccha, Milesia, Hammerschmidtia, Brachyopa, and 
Microdon. 
These tergites are moderately to feebly chitinized in the following 
genera: Pipiza s. l., Chrysogaster, Svrphus, Catabomba, Didea, Chryso- 
toxum, Melanostoma, Pyrophaena, Sphegina, Baccha, Khingia, Eristalis, 
Helophilus, Tropidia, Xvylota, Calliprobola, Eumerus, Syritta, Seri- 
comyia, Condidea, Spilomyia, Teuchocnemis, Triodonta, Somula, Crior- 
hina, -Blera, Crioprora, Mallota, Merodon, Temnostoma, Ceriodes, 
Voliicella, Copestylum, Mixogaster. 
