The tracheal system of libellulid nymphs is not closed; on the other hand, in the fully-grown nymphs the anterior stigmata occurring on the dorsal side are large, and the tracheæ arising from them are thick. These stigmata are permeable by the air. In half-grown and still younger stages of Æschna the two anterior thoracic stigmata are undeveloped. In order to breathe, the fully-grown nymph either rises up on the upper side and elevates the end of the body to the surface in order to take the air into the rectum, or it rests with the back of the thorax at the surface in order to breathe through the large stigmata. The young nymphs take in air only through the rectum. The young nymphs of Libellula and its allies, on the other hand, possess large thoracic stigmata, but they prefer to breathe through the rectum. The fully-grown nymphs of Agrion breathe through the thoracic stigmata. (Dewitz, in Kolbe.)
The position and number of pairs of stigmata.—The spiracles are usually situated in the soft membrane between the tergites and pleurites, but their exact position varies in different groups. In the Coleoptera they occupy on the thorax a more ventral position, and on the abdomen are placed near the edge of the dorsal side, under the elytra. In the dragon-flies, the first pair is situated much more dorsally than the second and third pairs; the following seven pairs are almost wholly ventral and lie concealed in the membranous fold near the external plate. In the Hemiptera, also, the abdominal stigmata, though entirely free and visible, are situated ventrally.
Primarily, in the embryo a pair of stigmata appear on each segment of the thorax and abdomen, except the 10th and 11th, and even possibly in the head, for a pair of stigmata are said to occur in the head of Podurids (Smynthurus) (Lubbock), though this statement needs confirmation. Scolopendrella, however, is known to possess a pair of cephalic spiracles.
From the foregoing statement it will be seen that while in existing winged insects no more than 10 (in Japyx 11) pairs of stigmata are to be found in any one species, yet that 12 segments of the body, in different groups taken collectively, bear them. The primitive number of pairs of spiracles, therefore, in winged insects, was 12, i.e. a pair in each thoracic segment, and a pair in each of the first nine abdominal segments. Insects were originally all holopneustic, and gradually as the type became differentiated into the different orders they became peripneustic or amphipneustic, and, in certain aquatic forms, apneustic. (See pp. 459, 461.)
In the still more primitive, probably wingless, ancestors of insects there was a larger number of stigmata. Hatschek, in 1877, discovered a pair of tracheal invaginations in each of the three posterior head-segments of the embryo of a moth, with stigmatal openings in the 1st and 2d maxillary segments.
Thus early in embryonic life every segment of the body, except those bearing the eyes and the last abdominal, bore a pair of stigmata, so that the primitive insect had at least 15, and perhaps more, pairs of stigmata.
The position of the stigmata is subject to much variation, the result of adaptation to this or that mode of life. Examples are those insects which live in dusty situations or usually more or less concealed in the earth, as in most beetles, and in the Hymenoptera. In such beetles, the stigmata are situated in the thin membrane between the segments; in the Hymenoptera, on the upper edge of the segments. In the Siphonaptera, Pediculina, bed-bug, and similar forms, which breathe an air freer from dust, the spiracles lie free on the outside of the body.
“When the stigmata are free and without any protection on the abdomen, there are other ways by which the entrance of foreign bodies into the tracheæ is prevented. In such cases the body is covered with dense hairs, as in most Diptera and Neuroptera, as well as many Lepidoptera; or there is situated in front of the stigma either a small fissure which is covered over by a number of hairs arising from the edge, as in many Orthoptera; or, as in most insects, a luxurious growth of hairs on the inside of the stigma forms a thick filter for the air. Thus we see that also in this respect each species of insect is completely adapted to its surroundings.” (Krancher.)
Fig. 404.—A, thoracic stigma of the house-fly: Sb, valve which closes the opening.