6 Siphonophora rosarum 
remain in a passive, uncontracted state, the contraction of the inner 
dorso-ventral muscles will have the effect of elevating the wing, because 
the pulling down of the “ crowning arch ” of the thorax will cause a pull 
to be exerted upon the upper portion of the alar membrane, and the 
junction of the lower portion of the alar membrane acting as the fulcrum, 
the wings will be elevated. These muscles may therefore be called the 
“ Elevator Muscles.” Coming now to the longitudinal muscles, it is 
probable that they perform, or assist in the performance of, some 
depressor function. The dorsal portion of the thorax being so strongly 
chitinized, will not allow of much movement of a telescopic character, 
so that the contraction of the longitudinal muscles—which, it will be 
remembered, are attached to the anterior and posterior faces of this 
dorsal arch—will have the effect of decreasing the distance between 
these faces; and as telescopic contraction is not possible, the only 
alternative is that the transverse measurements of this portion of the 
thorax will be increased. This can be illustrated in a concrete manner 
with the half of an india-rubber ball. If the ball be compressed 
along one diameter—which will represent the longitudinal axis of 
the thorax—the length of the diameter at right angles to the line of 
compression—representing the transverse axis of the thorax—will be 
increased. The effect of the increase in length of the transverse axis 
of the thorax will be to carry the point of attachment of the dorsal 
portion of the alar membrane beyond the point of attachment of the 
ventral portion. The total effect being that the wings will be 
depressed. The insect therefore possesses two sets of muscles for 
depressing the wings, and one set for elevating them. The actual 
sequence of contractions is probably as follows: First the small muscles 
attached to the insertions of the wings will cause the wings to leave 
their inverted, folded-up position over the back, and arrange themselves 
at right angles to the longitudinal axis of the body; the whole of the 
longitudinal muscles and the depressor muscles will then contract, 
imparting to the wings a powerful down-stroke. These muscles then 
relaxing, the elevator muscles will contract and raise the wings to their 
former position, and the cycle will begin again. 
The Alimentary Canal. 
The alimentary canal of the winged stage of S. rosarum (Text 
Figs. 1 and 2) exhibits some points of interest. The thorax of the 
insect is almost completely filled up with the wing muscles, and as a 
