90 MOSQUITOES OF NORTH AMERICA 
addition there are transverse connecting tubules in each segment, which however 
are not in the form of a simple bridge but formed like the two links of a rec- 
tangular triangle. In each segment branches go out from the longitudinal 
trunks which ramify in the manner of capillaries and twine about all the organs. 
The foremost ends, which are bent like knees, divide into several branches of 
which two on each side pass into the head and there take a branching de- 
velopment. 
“ From the penultimate segment two small trunks go to the last segment. 
“ At the point where the colon begins a countless number of weak, small 
tracheal branches arise which run to the colon, where they resolve themselves 
upon the previously described papille into innumerable branchlets. Haller, who 
also saw these fine tubes, explains them as an air reservoir. This, however, with- 
out considering that this would be a most unfavorable reservoir, appears falla- 
cious because the branches are exceedingly fine and in comparison with the two 
large tracheal trunks can only hold a very small amount of air. 
“ Histologically nothing remarkable can be said of the trachew. Inwardly, 
bounding the lumen, lies the chitinous integument showing the spiral threads; 
around it and in intimate connection with the fat body lies the peritoneal en- 
velope. Itis remarkable that the traches of the colon are without spiral threads. 
“The already frequently mentioned larval organ arising from the penulti- 
mate segment, the siphon, shows the following conditions: The two tracheal 
trunks pass upward to the right and left inside the tube; towards the end they 
lose their spiral-thread structure and become firm cylindrical cups which are 
differentiated by a considerable constriction at their bases. Between them lies 
a stiff, hollow, chitinous rod which unites with the two cups where they come to- 
gether and helps to bound the hollow space thus formed; thus this hollow space 
is formed by the cups on the right and left and by the hollow rod above and below. 
“Thus the two tracheal trunks do not empty separately to the outside, as 
Haller asserts, but through a single common orifice. 
“On the lower side the part of the wall of the common hollow space, which 
arises from the firm chitinous tube, is continued as two flaps which rest upon the 
siphon. The outer basal margins of these flaps are connected with the siphon by 
an elastic membrane. On the dorsal side the wall of the common hollow bears a 
median and two small flaps which are also connected with the end of the breath- 
ing tube by flexible membrane. 
“Tf now, by the contraction of the muscles pertaining to this apparatus, the 
entire upper tract is pulled down the result is twofold: first the rigid end-cups 
(of the tracheal tubes) are pressed down upon the constriction and bring about 
a closure at this point; secondly by the traction upon the chitinous rod, and 
through the muscles of the flaps, the flaps are brought together in the form of a 
three-sided pyramid composed of the three large flaps. When the apparatus is 
closed and forms, as it were, a pyramid resting with its base upon the tube, the 
two small dorsal flaps lie against the ventral flaps. 
“ When the larva wishes to breathe air, it sticks the end of the tube out of the 
water and opens the flaps. Thereby a wreath-like formation results which 
enables the animal to remain suspended from the water surface. 
“The muscles of the closing apparatus originate in the penultimate body- 
segment and remain free in their passage through the breathing tube. In all 
five pairs of these muscles are present. Two pairs belong to the ventrally located 
flaps, two pairs to the flaps lying opposite, and one pair belongs to the median 
chitinous rod. 
* * * * * * * * 
“The anus is surrounded by four lanceolate tubes which are to be looked upon 
as tracheal gills. They have an extremely fine and delicate cuticle under which 
