A. J. Grove 
5 
may be divided primarily into two distinct sets; one set taking up an 
obliquely dorso-ventral position, and the other stretching in a longi¬ 
tudinal horizontal direction along the dorsal portion of the thorax. 
The dorso-ventral muscles consist of four pairs—two pairs on each 
side of the body. Considering now only the two pairs of muscles of one 
side, it will be seen (Plate II, Fig. 2) that they occupy the greater part of 
the half of the thoracic cavity in which they lie, and are arranged with 
one pair internal to the other; and all of them are markedly oblique— 
antero-posteriorly and laterally—in their direction, so that in a trans¬ 
verse section thx-ough the middle of the thorax, all four belonging to 
that side will appear (see Plate II, Fig. 2). The outer pair—one, of 
course, belonging to each of the wings of the side under considera¬ 
tion—may be said to have their origin in the chitin of the floor of the 
thoracic cavity, and are inserted into the inflexed portion of the thoracic 
wall which occurs at the insertion of the wings (Plate II, Fig. 3). 
The inner pair have their origin in the floor of the thorax, a little 
nearer to the mid-ventral line than the previously mentioned pair, 
and are inserted into the dorsal wall of the thorax, where the dorsal 
surface merges into the lateral one. The longitudinal horizontal 
muscles all lie in the dorsal portion of the thoracic cavity, and 
consist in all of eight separate muscles (see Plate II, Fig. 2), 
arranged in four pairs—one pair above another. Taking them in 
order from above downwards, the first pair lie closely applied to the 
under side of the dorsal wall of the thorax ; each muscle lying a little to 
the right or left of the median dorsal line. The remaining three pairs 
are grouped together, are more deeply seated than the preceding pair, 
and lie much closer together. All these muscles are attached at their 
anterior ends to the front margin of the prothorax, and posteriorly to the 
hind wall of the metathorax. 
The action of the muscles. In order to make the action of the 
wing muscles quite clear, it will be advisable to consider first the action 
of each muscle individually, and then consider their action as a whole. 
Commencing with the outer pair of dorso-ventral muscles on each side, 
it will be obvious from Plate II, Figs. 2 and 3, that the contraction of 
these muscles will drag down the inflexed piece of chitin into which the 
muscle is inserted, and consequently exert a pull upon the lower portion 
of the alar membrane which is attached to it. Provided that the dorsal 
portion of the body wall remains rigid, the effect of this pull will be to 
depress the wing. These muscles may therefore be called the “ De¬ 
pressor Muscles.” Similarly, provided that the depressor muscles 
