Certain remarkable spines occur in limacodid larvæ, notably Empretia and Adoneta. These we have called caltrops spines, from their resemblance to the caltrops formerly used in repelling the attacks of cavalry. They are largely concerned in producing the poisonous and irritating effects resulting from contact with the caterpillars of these moths, and are situated in scattered groups near the end of the tubercles. A group of three is represented at Fig. 213, _e_. They are not firmly embedded in the cuticle, but on the contrary appear to become very easily loosened and detached, and they probably, when brought into contact with the skin of any aggressor, burrow underneath, and are probably in part the cause of the continual itching and annoyance occasioned by these creatures. It will be seen by reference to Fig. 213, _e′_, that the body of the spine is spherical, with one large, elongated, conical spine arising from it, the spherical base being beset with a number of minute, somewhat obtuse spinules. [Illustration: FIG. 214.—Glandular hairs of caterpillars. _A_, _Dasylophia anguina_: _a_, of body; _b_, of head; _c_, of prothoracic shield. _B_, _Ceratosia tricolor_: _a_, on body; _b_, on abdominal legs. _C_, _Schizura ipomeæ_: _a_, from third thoracic segment; _b_, from larva stage II; _c_, simple setæ from minute warts. ] =Glandular hairs and spines.=—In some insects occur fine, minute, hollow setæ from which exude, perhaps through pore-canals of extreme fineness, droplets of a clear watery or plasma-like sticky fluid. The club-shaped tenent hairs of the feet of Collembola, and the hairs fringing the feet of Diptera, are modified glandular hairs. Here they serve to give out a sticky fluid enabling the insect to walk on smooth surfaces; they end in a vesicle-like bulbous expansion, which may contain numerous pore-canals. Those of caterpillars were first noticed by Zeller, and Dimmock has particularly described those of the larvæ of Pterophoridæ. They are either club-shaped, or variously forked at the end (Fig. 214, _B_, _a_). They are usually replaced after the first larval moult by ordinary, simple, solid, pointed setæ, and their use in caterpillars is as yet unknown. Whether these hairs, as seems most probable, arise from a specialized glandular hypodermal cell, or not, has not yet been discovered. [Illustration: FIG. 215.—_A_, group of setæ arising from a subdorsal tubercle: _cut_, the cuticle; _hy_, the hypodermis; _sc_, the enlarged and specialized cells of the hypodermis which secrete the spines themselves; _pglc_, the nuclei which secrete the venomous fluid which fills the cavity of the seta (_s_), seen at _p_ in a broken spine. _B_, a short entire, and a long broken seta (_s-p_); _pgle_, four poison cells; _p_, the poison in the hollow of the spine. ]