All insects, in their perfect state, we are told, have three pairs of legs; but if you examine the under surface of certain butterflies, such as the Marbled White, or any of the Vanessas, Browns, or Heaths, it is quite likely that you will raise objection to such a statement; for in these you may possibly see only four legs. But this is the result of a too cursory observation. Look a little more closely at your specimen, and you will see a pair of smaller legs folded up under the fore part of the thorax. By means of a blunt needle you can straighten out these limbs, and then the difference in length to be observed between them and the other four is very striking indeed. They are also thinner than the middle and hind legs; and, unlike these, are not provided with claws. [Illustration: FIG. 8.--DIAGRAM OF THE WINGS OF A BUTTERFLY. I. _Fore wing._--1-5, subcostal nervules; 6, 7, discoidal nervules; 8-10, median nervules; 11, submedian nervure; 12, internal nervure; 13-15, disco-cellular nervules; 16, interno-median nervule; 17, median nervure; 18, subcostal nervure; _a_, costal nervure; _b_, costa or anterior margin; _c_, apex or anterior angle; _d_, posterior or hind margin; _e_, posterior or anal angle; _f_, interior or inner margin; _g_, base; _h_, discoidal cell. II. _Hind wing._--1, 2, subcostal nervules; 3, discoidal nervule; 4-6, median nervules; 7, submedian nervure; 8, precostal nervure; 9, subcostal nervure; 10, median nervure; 11, 12, disco-cellular nervules; _a_, costal nervure; _b_, costa or anterior margin; _c_, apex or anterior angle; _d_, hind margin; _e_, tail or caudal appendage; _f_, anal angle; _g_, abdominal or inner margin; _h_, base.] These imperfectly developed legs are, of course, quite useless as far as walking is concerned; indeed, it is extremely doubtful as to whether they are of any service whatever to the owner. On one occasion, however, while watching a Peacock Butterfly apparently engaged in cleaning its divided proboscis, I observed that this organ was frequently passed under the thorax, and that the front pair of legs were pressed against it on each side, while it was being drawn outward between them. It is probable, therefore, that these limbs constitute a pair of brushes by means of which the fine grooves of the divided trunk are cleared of any solid or sticky matter that may lodge therein. It is certain that moths, and those butterflies that possess six _equal_ legs, use the front pair for this same purpose. The former, also, employ them for brushing their antennae, which seem to be, by the way, particularly sensitive to different kinds of irritation. [Illustration: FIG. 9.--THE UNDEVELOPED FORE LEG OF A BUTTERFLY.]