ATTACHMENT OF THE ABDOMEN TO THE THORAX IN DIPTERA 273 
of species found in which the sternites were composed of membrane or 
very weak chitin. In so far as the tergites are concerned, these are of 
such a composition only in Orphnephila americana (Plate XIV, 18); while 
the following species show this condition in so far as the sternites are 
concerned: Simulium hirtipes (Plate XIV, 16), Orphnephila americana 
(Plate XIV, 18), Phora concinna (Plate XIX, 32), Gastrophilus intestinalis 
(Plate XXI, 37), Chlorops sp. (Plate XXX, 69), Melophagus ovinus 
(Plate XXXI, 73), and Olfersia americana (Plate XXXI, 74). In- 
stances of the first abdominal tergite remaining membranous, possibly to 
facilitate the movement of the caudal end of the abdomen, are not un- 
common. This is the case in Anopheles quadrimaculata (Plate XII, 10), 
Midas clavatus (Plate XVII, 27), Lonchoptera sp. (Plate XIX, 31), Pip- 
unculus atlanticus (Plate XX, 34), and Borborus equinus (Plate XXVI, 
57). Other species, such as Rhabdophaga strobiloides (Plate XIII, 14) and 
Drosophila melanogaster (?) (Plate XXX, 70), have a slight amount of 
chitin laid down in this sternite. 
The same forces that have had their effect in decreasing the size of the 
first abdominal tergites have produced the same result in the sternites 
of a larger number of species. Most decidedly is this the case in Tri- 
chocera brumalis (Plate X, 4, 1s), in Thereva fucata (Plate XVII, 25, 1s), 
in Scenopinus fenestralis (Plate XVII, 26, 1s), in Leptogaster loewi (Plate 
XVIII, 28, 1s), in Dolichopus cuprinus (Plate XVIII, 29, 1s), and in 
Muscina stabulans (Plate XXII, 41, 1s); while the same condition holds 
to a lesser extent in Leia winthemi (Plate XII, 12, 1s), in Sctara ochrolabis 
(Plate XIII, 13, 1s), in Hirmoneura sp. (Plate XVI, 22, 1s), and in 
Sarcophaga communis (Plate XXII, 40, Is). 
Subdivisions of primary sternites are found oftener than are subdivisions 
of primary tergites, the second sternite being the one oftenest modified 
in this way. In Pachyrrhina ferruginea (Plate IX, 3, 1s, and 1s,), in 
Plecia heteroptera (Plate XIII, 15, 2s, and 2s,), in Rhamphomyza sp. (Plate 
XVIII, 30, 2s, and 2s,), and in Calobata albiceps (Plate XXVIII, 63, 2s, 
and 2s,), the anterior part is separated from the posterior part of the 
second sternite by membrane. 
In only one species, Chrysops indus (Rlate XV, 20, 1s and 2s), were 
the first and second sternites found to have coalesced. Here a double 
row of sense pits marks the usual position of the suture between the first 
and the second segment. 
