Fig. 362.—Median section through the femoro-tibial joint of leg of Coccinella, showing at o the opening through which the blood oozes out; f, femur; t, tibia; e, extensor muscle of the tibia: s, sinew of the same; at ch, chitinized; h, articular membrane; v, tibial process.—After Lutz.
The two pairs of remarkably large, soft, eversible, forked, orange-yellow glands of the European genus Malachius, are thrust out from the side of the 1st and the 3d thoracic segments. They are everted by blood-pressure, and retracted by muscles. The larva of Hydrophilus piceus ejects by the anus a black, fœtid fluid.
Claus has shown that the larva of Lina populi and other Chrysomelidæ possess numerous minute, eversible glands in each of the warts on the upper surface of the body, each gland containing a whitish, repellent fluid smelling like the oil of bitter almonds and containing salicylic acid derived from its food-plant, which issues as pearl-like drops. Candèze thinks the fluid may contain prussic acid. The fluid is secreted by a variable number of glandular cells, each provided with an efferent duct. The larvæ of saw-flies, notably Cimbex americana, also eject droplets of a clear fluid from non-evaginable glands situated near each stigma (Chlolodkovsky).
The blood as a repellent fluid.—In this connection it may be mentioned that though there are no special glands present, many beetles emit drops of blood from the femoro-tibial joints of their legs as a means of defence. Such are the oil-beetles (Meloë), Cantharis, Lytta. The cantharadine secreted by these beetles, according to Beauregard, is an efficient means of defence, as birds, reptiles, and carnivorous insects will not usually attack them. This substance is formed in the blood and also in the genital organs, and is so extremely caustic that scavenger insects which feed upon their dead bodies will leave untouched the parts containing cantharadine, and if May-beetles or mole-crickets are washed with the blood of Meloë or with cantharidate of potassa, it will for several days render them safe from the attacks of the carabids which usually prey upon them. The eggs even after deposition are strongly vesicant, and are thus free from the attacks of egg-eating insects (Cuénot). The Coccinellidæ are also protected by a yellow, mucilaginous, disagreeable fluid oozing out of the ends of the femora; in our common, two-spotted lady-bird (C. bipunctata) the yellow fluid is disagreeable, smelling like opium. Lutz has found that the blood in Coccinellidæ passes out through a minute opening situated at the end of each femur (Fig. 362). The blood is very repellent to insectivorous animals.
The Dyticidæ eject from the anus a colorless, disagreeable fluid, while these beetles, and especially the Gyrinidæ, when captured send out a milky fluid which appears to issue from the joints of the body. The Silphidæ throw out both from the mouth and vent a fœtid liquid with an ammoniacal odor. They possess but a single anal gland, the reservoir opening on one side of the rectum (Dufour).
Other malodorous insects have not yet been investigated; such are the very persistent odors of lace-winged flies (Chrysopa).
More agreeable secretions, but probably formed by similar glands, is the odor of rose or hyacinth given out by Cicindelæ, or the rose fragrance exhaled by the European Aromia moschata.
Eversible glands of caddis-worms and caterpillars.—Gilson, while investigating the segmentally disposed thoracic glands of larval Trichoptera, has found in the larva of Limnophilus flavicornis that the sternal prothoracic tubercle gives exit to an underlying tubular gland. In Phryganea grandis each thoracic sternum affords an exit to an eversible gland. Many caterpillars, as our subjoined list will show, are very well protected by eversible repugnatorial glands situated either in the under or upper side of the body. Since the time of De Geer (1750) the fork-tailed larva of Cerura has been known to throw out a secretion, which was described by Bonnet in 1755 as a true acid, sharp, sour, and biting. This spraying apparatus in Cerura (Harpyia) vinula has been well described by Klemensiewicz (Fig. 366, 4), though Rengger in 1817 noticed the general form of the secretory sac, and that it opens out in two muscular eversible tubes, out of which the secretion is ejected.
The fork-tailed larva of Macrurocampa marthesia, which is much like that of Cerura, when teased sends out a jet of spray to the distance of nearly an inch from each side of the neck. While examining the very gayly-colored and heavily-spined caterpillars of Schizura concinna we observed that when a fully-grown one was roughly seized with the forceps or fingers it sent out a shower of spray from each side of the prothoracic segment, exactly like that of Cerura and Macrurocampa.
In the European Cerura vinula the apparatus consists of a single sac, which opens by a narrow transverse slit on the under side of the neck, out of which is rapidly everted four lateral tubes, two on each side (Fig. 366, 4, t), which are withdrawn within the opening by the contraction of several fine muscles. The apparatus in the American C. multiscripta is as in the European C. vinula. In a living specimen the large secretory sac was seen to be of the same size and shape as in Macrurocampa, and of the color of raw silk. The sac when distended extends back to a little behind the middle pair of legs, and is nearly two-thirds as wide as the body. The caterpillar sent out the fluid when handled, but we could not make it spray.