ae 
ATTACHMENT OF THE ABDOMEN TO THE THORAX IN DIPTERA 271 
to decrease in relative size. This one is usually nothing more than a 
narrow band as compared with those that follow. (Plates IX, 2 and 3, 
Pere Overt {- XViI, 22, XVII, 26, XVIII, 28 and 30, XX, 35.) Nor ean 
_all of this decrease be attributed to the failure of the hypodermal cells 
to deposit chitin in the membrane just cephalad of the first tergite. It 
seems very probable that the belief that one segment may gradually dis- 
appear owing to the great development of a contiguous segment, is some- 
times warranted. 
This theory of growth of one segment at the expense of an adjoining 
one might at first seem to be very good evidence for use in determining 
what has happened in case of species which show but one chitinized ter- 
gite to two spiracles, such as Lonchoptera sp. (Plate XIX, 31, 1t and 2t), 
Macrorchis ausoba (Plate XXII, 42, 1t and 2t), Euaresta festiva (Plate 
XXVII, 62, 1t and 2t), Calobata albiceps (Plate XXVIII, 63, 1t and 2t), 
Sepsis violacea (Plate XXVIII, 64, 1t and 2t), Pzophila casez (Plate X XVIII, 
65, 1t and 2t), and Sphyracephala brevicornis (Plate X XIX, 67, 1t and 2t). 
But on considering the large number of examples that might be regarded 
as transitional stages between the well-defined first and second tergite, on 
the one hand, and the single tergite to represent both, on the other hand, 
it is easier to believe in a fused condition of these two tergites than in 
the alternative which would permit the second, or larger, tergite to crowd 
out the first, or smaller, one altogether. These transitory stages may be 
seen in a number of species. In Platypeza velutina (Plate XIX, 33) there 
appear two notches in the latero-ventral margin of the tergites at about 
the place where one would expect to find a dividing suture, and only a 
lighter band can be seen extending up through the middle of the tergites. 
In Pipunculus atlanticus (Plate XX, 34) the only remaining sign of a 
suture is a very faint impressed line. In Tachina mella (Plate X XI, 38), 
only a lighter-colored band marks what may have been the position of a 
suture in its progenitors. Thisis likewise the case in Sarcophaga communis 
(Plate XXII, 40). Very faint impressed lines exist in Thelaira nigripes 
(Plate X XI, 39), in Muscina stabulans (Plate XXII, 41), and in Dictya 
umbrarum (Plate XXVI, 58). Latero-ventral incisions, sometimes with 
and sometimes without vestigial sutures, are to be seen in some species, 
as Sapromyza lupulina (Plate XXVI, 59), Parydra limpidipennis (Plate 
XXIX, 68), Chlorops sp. (Plate XXX, 69), and Drosophila melanogaster 
(?) (Plate XXX, 70). A vestigial suture exists in Rivellia viridulans 
(Plate XXVII, 60), while only a bit of membrane is to be found in such 
