Dewitz confirmed the opinion of Blackwell, that a glutinous liquid is exuded from the apices of the tenent hairs which fringe the empodium. By fastening insects feet uppermost on the under side of a covering glass which projects from a glass slide, the hairs which clothe the empodia of the foot of a fly (Musca erythrocephala) may be seen to be tipped with drops of transparent liquid. On the leg being drawn back from the glass, a transparent thread is drawn out, and drops are found to be left on the glass. In cases where these hairs are wanting, as in the Hemiptera, the adhesive fluid exudes directly from pores in the foot. In the beetles (Telephorus dispar) and other insects the tenent hairs on the foot end in sharp points, below which are placed the openings of the canals. The glands, Dewitz states, are chiefly flask-shaped and unicellular, situated in the hypodermis of the chitinous coat; each gland opening into one of the hairs (Fig. 108); they are each invested by a structureless tunica propria, and contain granular protoplasm, a nucleus placed at the inner side, and a vesicle, prolonged into a tube which, traversing the neck of the gland, is attached to the root of the hair; the vesicle receiving the secretion. Each gland is connected with a fine nerve-twig, and secretion is probably voluntary. Among the tenent hairs of the empodium are others which must be supplied with a nerve, forming tactile hairs, as they each proceed from a unicellular ganglion (Fig. 108, n″). The secretion is forced out of the gland by the contraction of the protoplasm, Dewitz having seen the secretion driven out from the internal vesicle into its neck.
Fig. 128.—The walk of an orthopterous insect: series to be followed from right to left.—After Marey.
Fig. 129.—Beetle walking: series to be followed from left to right.—After Marey.
Fig. 130.—A, end of an adhesive hair of a weevil (Eupolus): i′, canal: i‴, its external opening at the end of the hair. B, end of a similar hair of Telephorus with drops of the secretion.—After Dewitz.
In the spherical last tarsal joint of Orthoptera (Fig. 109), which is without these tenent hairs, nearly all the cells of the hypodermis are converted into unicellular glands, each of which sends out a long, fine, chitinous tubule, which is connected with its fellows by very fine hairs and is continuous with the chitinous coat of the foot and opens through it. The sole of the foot is elastic and adapts itself to minute inequalities of surfaces, while the anterior of each tarsal joint is almost entirely occupied by an enlargement of the trachea, which acts on the elastic sole like an air chamber, rendering it tense and at the same time pliant. Dewitz adds that the apparatus situated on the front legs of the male of Stenobothrus sibiricus (Fig. 131) must have the function of causing the legs to adhere closely to the female by the excretion of an adhesive material. The hairs of the anterior tarsi of male Carabi also appear to possess the power of adhesion. In the house-fly the empodia seem to be only called into action when the insect has to walk on vertical smooth surfaces, as at other times they hang loosely down.
Burmeister observed the use of a glutinous secretion for walking in dipterous larvæ, and Dewitz found that the larva of a Musca used for this purpose a liquid ejected from the mouth. The larvæ of another fly (Leucopis puncticornis) perform their loop-like walk by emitting a fluid from both mouth and anus. A Cecidomyia larva is able to leap by fixing its anterior end by means of an adhesive fluid. The larva of the leaf-beetle, Galeruca, moves by drawing up its hinder end, fixing it thus, and carrying the anterior part of the body forward with its feet until fully extended, when it breaks the glutinous adhesion. The abdominal legs of some saw-fly larvæ have the same power.
Dahl could not detect in the foot of the hornet (Vespa crabro) any space which could be considered as a vacuum.