In those Arachnida which are provided with poison-glands, these scent-glands are absent, but in certain Acarina and Linguatulidæ, which have no poison-glands, there are various oil-glands, stigmatic glands, as well as scent-glands, and in seizing a Thelyphonus with the forceps we have observed it to send out from each side of the body a jet of offensive spray.
We not infrequently find in myriopods (Polydesmidæ, Julidæ, and Glomeris) repugnatorial or the so-called cyanogenic glands, which are either paired, opening on the sides of the body, or form a single row along the median line of the under side of the body. Leidy describes and figures the spherical glands of Julus marginatus, of which there are 50 pairs. These glands have been regarded as modified nephridia, but are more probably coxal glands, and the homologues of the parapodial glands of annelid worms.
Fig. 358.—Sternite of Machilis maritima, with the pair of coxal sacs (cb) on the right side everted; hs, coxal appendages; m, retractor muscles.—After Oudemans, from Lang.
Eversible coxal glands.—True coxal glands occur in Scolopendrella immaculata on the 2d to 11th segment, on the inner side of the base of the legs (Fig. 15, c.g.). Homologous glands also occur in the same position in Campodea staphylinus (also in C. cookei and C. mexicana) on the 1st to 8th abdominal segments, and Oudemans has described a pair of eversible sacs on each side of segments one to seven of Machilis. These eversible sacs in the synapterous insects are evidently modified coxal glands, and are probably repugnatorial as well as respiratory in function.
The apparatus consists of an eversible gland, composed of hypodermic cells, usually retracted by a slender muscle and with an efferent passage, but the glands vary greatly in shape and structure in different insects. In some cases these fœtid glands appear not to be the homologues of the coxal glands, but simply dermal glands.
These repugnatorial glands are of not infrequent occurrence in the lower or more generalized winged insects, and in situation and appearance are evidently the homologues of the coxal glands of the Symphyla and Synaptera.
Fœtid glands of Orthoptera.—In the ear-wigs (Forficula and Chelidura) Meinert has detected a pair of what he calls fœtid glands at the posterior margin of the dorsal plates of the 2d and 3d abdominal segments.
Vosseler also describes the same glands as consisting of a retort-shaped sac, in whose walls are numerous small hypodermal cells and large single glandular cells provided with an efferent passage, the fluid being forced out by the pressure of the dermal muscles, one acting specially to retract the gland. The creature can squirt to a distance of 5 and even 10 cm. (4 inches) a yellowish-brown liquid or emulsion with the odor of a mixture of carbolic acid and creosote.
The large eversible dorsal glands of the Blattidæ, since they contain numerous hairs, which, when everted, are fan-like or like tufts, serve, as in the spraying or scent apparatus, to disseminate the odor, and might be classified with the alluring unicellular scent-glands or duftapparat of other insects, as they are by some authors; but as the glands are large and compound they may prove to be the homologues of the coxal glands rather than of the dermal glands.