In their claw or leg-like form these male antennae also repeat in the head, the general form of the legs, whose prehensile and grasping functions they a.s.sume. We have seen above that the appendages of the head and thorax are alike in the embryo, and the present case is an interesting example of the unity of type of the jointed appendages of insects, and articulates generally.
[Ill.u.s.tration: 120. Antennae of Goniodes.]
Another point of interest in these degraded insects is, that the process of degradation begins either late in the life of the embryo or during the changes from the larval to the adult, or winged state. An instance of the latter may be observed in the wingless female of the canker worm, so different from the winged male; this difference is created after the larval stage, for the caterpillars of both s.e.xes are the same, so far as we know. So with numerous other examples among the moths. In the louse, the embryo, late in its life, resembles the embryos of other insects, even Corixa, a member of a not remotely allied family. But just before hatching the insect a.s.sumes its degraded louse physiognomy. The developmentist would say that this process of degradation points to causes acting upon the insect just before or immediately after birth, inducing the retrogression and r.e.t.a.r.dation of development, and would consider it as an argument for the evolution of specific forms by causes acting on the animal while battling with its fellows in the struggle for existence, and perhaps consider that the metamorphoses of the animal within the egg are due to a reflex action of the modes of life of the ancestors of the animal on the embryos of its descendants.
[Ill.u.s.tration: 125. The Turkey Louse.]
FOOTNOTES:
[Footnote 5:
Ha! whare ye gaun, ye crowlin ferlie!
Your impudence protects you sairly: I canna say but ye struift rarely, Owre gauze and lace; Tho" faith, I fear ye dine but sparely On sic a place.
Ye ugly, creepin, blastic wormer, Detested, shunn"d by saunt and sinner, How dare ye set your fit upon her Sae fine a lady!
Gae somewhere else and seek your dinner On some poor body.
(To a Louse.--Burns.)]
[Footnote 6: We notice while preparing this article that a journal of Parasitology has for some time been issued in Germany--that favored land of specialists. It is the "Zeitschrift fur Parasitenkunde," edited by Dr. E. Hallier and F A. Zurn. 8vo, Jena.]
[Footnote 7: Figure 111 represents the parts of the mouth in a large specimen of _Pediculus_ vestimenti, entirely protruding, and seen from above, magnified one hundred and sixty times; aa, the summit of the head with four bristles on each side; _bb_, the chitinous band, and _c_, the hind part of the lower lip, such as they appear through the skin by strong transmitted light; _dd_, the foremost protruding part of the lower lip (the haustellum); _ee_, the hooks turned outwards; _f_, the inner tube of suction, slightly bent and twisted; the two pairs of jaws are perceived on the outside as thin lines; a few blood globules are seen in the interior of the tube.]
CHAPTER X.
THE DRAGON FLY.
Were we to select from among the insects a type of all that is savage, relentless, and bloodthirsty, the Dragon fly would be our choice. From the moment of its birth until its death, usually a twelve-month, it riots in bloodshed and carnage. Living beneath the waters perhaps eleven months of its life, in the larva and pupa states, it is literally a walking pitfall for luckless aquatic insects; but when transformed into a fly, ever on the wing in pursuit of its prey, it throws off all concealment, and reveals the more unblushingly its rapacious character.
Not only do its horrid visage and ferocious bearing frighten children, who call it the "Devil"s Darning-needle," but it even distresses older persons, so that its name has become a byword. Could we understand the language of insects, what tales of horror would be revealed! What traditions, sagas, fables, and myths must adorn the annals of animal life regarding this Dragon among insects!
To man, however, aside from its bad name and its repulsive aspect, which its gay trappings do not conceal, its whole life is beneficent. It is a scavenger, being like that cla.s.s ugly and repulsive, and holding literally, among insects, the lowest rank in society. In the water, it preys upon young mosquitoes and the larvae of other noxious insects. It thus aids in maintaining the balance of life, and cleanses the swamps of miasmata, thus purifying the air we breathe. During its existence of three or four weeks above the waters, its whole life is a continued good to man. It hawks over pools and fields and through gardens, decimating swarms of mosquitoes, flies, gnats, and other baneful insects. It is a true Malthus" delight, and, following that sanguinary philosopher, we may believe that our Dragon fly is an entomological Tamerlane or Napoleon sent into the world by a kind Providence to prevent too close a jostling among the myriads of insect life.
We will, then, conquer our repugnance to its ugly looks and savage mien, and contemplate the hideous monstrosity,--as it is useless to deny that it combines the graces of the Hunchback of Notre Dame and d.i.c.kens"
Quilp, with certain features of its own,--for the good it does in Nature.
Even among insects, a cla.s.s replete with forms the very incarnation of ugliness and the perfection of all that is hideous in nature, our Dragon fly is most conspicuous. Look at its enormous head, with its beetling brows, retreating face, and heavy under jaws,--all eyes and teeth,--and hung so loosely on its short, weak neck, sunk beneath its enormous hunchback,--for it is wofully round-shouldered,--while its long, thin legs, shrunken as if from disease, are drawn up beneath its breast, and what a hobgoblin it is!
Its gleaming wings are, however, beautiful objects. They form a broad expanse of delicate parchment-like membrane drawn over an intricate network of veins. Though the body is bulky, it is yet light, and easily sustained by the wings. The long tail undoubtedly acts as a rudder to steady its flight.
These insects are almost universally dressed in the gayest colors. The body is variously banded with rich shades of blue, green, and yellow, and the wings give off the most beautiful iridescent and metallic reflections.
During July and August the various species of Libellula and its allies most abound. The eggs are attached loosely in bunches to the stems of rushes and other water-plants. In laying them, the Dragon fly, according to Mr. P. R. Uhler"s observations, "alights upon water-plants, and, pushing the end of her body below the surface of the water, glues a bunch of eggs to the submerged stem or leaf. Libellula auripennis I have often seen laying eggs, and I think I was not deceived in my observation that she dropped a bunch of eggs into the open ditch while balancing herself just a little way above the surface of the water. I have, also, seen her settled upon the reeds in brackish water with her abdomen submerged in part, and there attaching a cl.u.s.ter of eggs. I feel pretty sure that L. auripennis does not always deposit the whole of her eggs at one time, as I have seen her attach a cl.u.s.ter of not more than a dozen small yellow eggs. There must be more than one hundred eggs in one of the large bunches. The eggs of some of the Agrions are bright apple-green, but I cannot be sure that I have ever seen them in the very act of oviposition. They have curious habits of settling upon leaves and gra.s.s growing in the water, and often allow their abdomens to fall below the surface of the water; sometimes they fly against the surface, but I never saw what I could a.s.sert to be the projecting of the eggs from the body upon plants or into the water. The English entomologists a.s.sert that the female Agrion goes below the surface to a depth of several inches to deposit eggs upon the submerged stems of plants." The Agrions, however, according to Lucaze Duthiers, a French anatomist, make, with the ovipositor, a little notch in the plant upon which they lay their eggs.
[Ill.u.s.tration: 127. Under side of head of Diplax, with the labium or mask fully extended. _x_, _x_", _x_"the three subdivisions of the labium. _y_, the maxillae or second pair of jaws.]
These eggs soon hatch, probably during the heat of summer. The larva is very active in its habits, being provided with six legs, attached to the thorax, on the back of which are the little wing-pads, or rudimentary wings. The large head is provided with enormous eyes, while a pair of simple, minute eyelets (ocelli) are placed near the origin of the small bristle-like feelers, or antennae. Seen from beneath, instead of the formidable array of jaws and accessory organs commonly observed in most carnivorous larvae, we see nothing but a broad, smooth mask covering the lower part of the face; as if from sheer modesty our young Dragon fly was endeavoring to conceal a gape. But wait a moment. Some unwary insect comes within striking distance. The battery of jaws is unmasked, and opens upon the victim. This mask (Fig. 127) is peculiar to the young, or larva and pupa of the Dragon fly. It is the labium, or under lip greatly enlarged, and armed at the broad spoon-shaped extremity (Fig.
127, _x_) with two sharp hooks, adapted for seizing and retaining its prey. At rest, the terminal half is so bent up as to conceal the face, and thus the creature crawls about, to all appearance, the most innocent and lamb-like of insects.
[Ill.u.s.tration: 128. Abdominal valves; _a_, side view.]
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 tracheae are here collected in a large ma.s.s, sending their branches into folds of membrane lining the end of the alimentary ca.n.a.l, and which act like a piston to force out the water. The entrance to the ca.n.a.l is protected by three to five triangular h.o.r.n.y 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.
[Ill.u.s.tration: 129. Agrion; _b_, False Gill of Larva.]
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 tracheae, or air-vessels. They are not true gills, however, as the blood is not aerated in them. They only absorb air to supply the tracheae, 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.
[Ill.u.s.tration: 130. Pupa of Cordulia.]
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 aeschnas 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.
[Ill.u.s.tration: 131. Pupa of Didymops?]
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 a.s.sume 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.
[Ill.u.s.tration: 132. Pupa of aeschna.]
When about to change into the adult fly, the pupa climbs up some plant near the surface of the water. Again its back yawns wide open, and from the rent our Dragon fly slowly emerges. For an hour or more, it remains torpid and listless, with its flabby, soft wings remaining motionless.
The fluids leave the surface, the crust hardens and dries, rich and varied tints appear, and our Dragon fly rises into its new world of light and sunshine a gorgeous, but repulsive being. Tennyson thus describes these changes in "The Two Voices":--
To-day I saw the Dragon fly Come from the wells where he did lie.
An inner impulse rent the veil Of his old husk: from head to tail Came out clear plates of sapphire mail.
He dried his wings; like gauze they grew; Through crofts and pastures wet with dew A living flash of light he flew.
Of our more common, typical forms of Dragon flies, we figure a few, commonly observed during the summer. The three-spotted Dragon fly (Libellula trimaculata), of which figure 133 represents the male, is so called from the three dark clouds on the wings of the female. But the opposite s.e.x differs in having a dark patch at the front edge of the wings, and a single broad cloud just beyond the middle of the wing.
Libellula quadrimaculata, the four-spotted Dragon fly (Fig. 134), is seen on the wing in June, flying through dry pine woods far from any standing water.
[Ill.u.s.tration: 133. Libellula trimaculata, male.]
[Ill.u.s.tration: 134. Libellula quadrimaculata.]
The largest of our Dragon flies are the "Devil"s Darning-needles,"
Eschna heros and grandis, seen hawking about our gardens till dusk. They frequently enter houses, carrying dismay and terror among the children.
The hind-body is long and cylindrical, and gaily colored with bright green and bluish bands and spots.
[Ill.u.s.tration: 135. Diplax Berenice, male.]
[Ill.u.s.tration: 136. Diplax Berenice, female.]
[Ill.u.s.tration: 137. Larva of Diplax.]