Not only does the immature Dragon fly walk over the bottom of the pool or stream it inhabits, but it can also leap for a considerable distance, and by a most curious contrivance. By a syringe-like apparatus lodged in the end of the body, it discharges a stream of water for a distance of two or three inches behind it, thus propelling the insect forwards. This apparatus combines the functions of locomotion and respiration. There are, as usual, two breathing pores (stigmata) on each side of the thorax. But the process of breathing seems to be mostly carried on in the tail. The tracheæ are here collected in a large mass, sending their branches into folds of membrane lining the end of the alimentary canal, and which act like a piston to force out the water. The entrance to the canal is protected by three to five triangular horny valves (Fig. 128, 9, 10, 128 a, side view), which open and shut at will. When open, the water flows in, bathing the internal gill-like organs, which extract the air from the water, which is then suddenly expelled by a strong muscular effort.
In the smaller forms, such as Agrion (A. saucium, Fig. 129; Fig. 129 b, side view of false gill, showing but one leaf), the respiratory leaves, called the tracheary, or false-gills, are not enclosed within the body, but form three broad leaves, permeated by tracheæ, or air-vessels. They are not true gills, however, as the blood is not aerated in them. They only absorb air to supply the tracheæ, which aerate the blood only within the general cavity of the body. These false gills also act as a rudder to aid the insect in swimming.
It is interesting to watch the Dragon flies through their transformations, as they can easily be kept in aquaria. Little, almost nothing, is known regarding their habits, and any one who can spend the necessary time and patience in rearing them, so as to trace up the different stages from the larva to the adult fly, and describe and figure them accurately, will do good service to science.
130. Pupa of Cordulia.
131. Pupa of Didymops?
Mr. Uhler states that at present we know but little of the young stages of our species, but the larva and pupa of the Libellulas may be always known from the Æschnas by the shorter, deeper and more robust form, and generally by their thick clothing of hair. Figure 130 represents the pupa of Cordulia lateralis, and figure 131 that of a Dragon fly referred doubtfully to the genus Didymops. For descriptions and figures of other forms the reader may turn to Mr. Louis Cabot's essay "On the Immature State of the Odonata," published by the Museum of Comparative Zoology at Cambridge.
The pupa scarcely differs from the larva, except in having larger wing-pads (Fig. 132). It is still active, and as much of a gourmand as ever. When the insect is about to assume the pupa state, it moults its skin. The body having outgrown the larva skin, by a strong muscular effort a rent opens along the back of the thorax, and the insect having fastened its claws into some object at the bottom of the pool, the pupa gradually works its way out of the larva-skin. It is now considerably larger than before. Immediately after this tedious operation, its body is soft, but the crust soon hardens. This change, with most species, probably occurs early in summer.