Sas a ee ee 
M.H. Rathke on the Respiratory Process in Insects. 85 
ment was proper to the air-vessels themselves, but at last he 
perceived that several extremely delicate thread-worms issued 
from these vessels ; others twisted about in some of the air-sacs 
of the abdomen. 
§4. In the Tabanide, the different genera and species of which 
have a more or less depressed abdomen, the upper plate of the 
abdominal segment is somewhat larger than the lower one, and 
consequently assists more or less in the formation of the lateral 
walls of the abdomen, with which the lower plates have little or 
nothing to do. The thinner and softer portion of the skin in- 
tervening between the upper and lower plates is always of con- 
siderable breadth ; it is always far broader at its anterior than 
at its posterior end, and possesses great extensibility and elas- 
ticity. 
se the author’s investigations of several species, and espe- 
pecially of Zabanus autumnalis, the respiratory movements of 
these animals are effected by the movement of the lower abdo- 
minal plates (especially those of the anterior segments) to and 
from the upper ones, during which the softer lateral portions 
appear alternately broader and narrower. But when the respi- 
ration is increased in force, the ends of the upper plates of the 
anterior segments, which assist in forming the lateral walls of 
these segments, are somewhat drawn inwards and then again 
pushed out. With a still more powerful respiration the lower 
plates are pushed a little over each other, and all drawn a very 
little towards the breast ; so that the lower surface of the abdo- 
men is somewhat shortened, but immediately returns to its for- 
mer position. 
In the Tabanida, as in the Beetles, peculiar muscles exist only 
for the contraction of the abdominal cavity, and these muscles 
coincide almost exactly in position and attachment with those of 
the Beetles. All these muscles are, however, both absolutely and 
relatively, very delicate and thin. 
At the base of the abdomen there are two large air-sacs; the 
tracheze are only of moderate width, and shrub-like in form, 
The canal which unites the tracheal stems on each side is also 
only of moderate size. 
§ 5. In Empis the inferior pieces of the abdominal segments 
are comparatively smaller than in Tabanus ; they form the eutire 
ventral wall of the abdomen. Only the three or four anterior ones 
rise and sink alternately, but very weakly and scarcely percep- 
tibly. The abdomen also frequently curves a little downwards. 
In the Tipularia also the lower pieces of the three or four an- 
terior segments, which are comparatively still narrower, move up 
and down scarcely perceptibly and but seldom. No movement 
that could be regarded as connected with respiration was de- 
