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 w4iere 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 papillas 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 tracheae. 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. It is remarkable that the tracheae 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. 



" If 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 fiaps, 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. 



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" 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 



