[Illustration FIG. 32.--Interior view of head of milkweed butterfly: _cl_, clypeus; _cor_, cornea of the eye; _oe_, oesophagus, or gullet; _fm_, frontal muscle; _dm_, dorsal muscles; _lm_, lateral muscles; _pm_, muscles moving the palpus (Burgess).]
[Illustration FIG. 33.--Labial palpus of _Colias_, magnified 10 diameters.]
It will be observed from what has been said that the head in these creatures is to a large extent the seat of the organs of sense and alimentation. What the function of the antennæ may be is somewhat doubtful, the opinion of scientific men being divided. The latest researches would indicate that these organs, which have been regarded as the organs of smell and sometimes as the organs of hearing, have probably a compound function, possibly enabling the creature to hear, certainly to smell, but also, perhaps, being the seat of impressions which are not strictly like any which we receive through our senses.
[Illustration FIG. 34.--_Colias philodice_: _a_, antenna; _p_, extremity of palpus; _pl_, prothoracic leg; _ml_, mesothoracic leg; _hl_, metathoracic or hind leg; _t_, proboscis.]
_Thorax._--The thorax is more or less oval in form, being somewhat flattened upon its upper surface. It is composed of three parts, or segments, closely united, which can only be distinguished from one another by a careful dissection. The anterior segment is known as the prothorax, the middle segment as the mesothorax, and the after segment as the metathorax. The legs are attached in pairs to these three subdivisions of the thorax, the anterior pair being therefore sometimes spoken of as the prothoracic legs, the second pair as the mesothoracic legs, and the latter pair as the metathoracic legs (Fig. 34). On either side of the mesothorax are attached the anterior pair of wings, over which, at their insertion into the body, are the _tegulæ_, or lappets; on either side of the metathorax are the posterior pair of wings. It will be seen from what has been said that the thorax bears the organs of locomotion. The under side of the thorax is frequently spoken of by writers, in describing butterflies, as the _pectus_, or breast.
_The Abdomen._--The abdomen is formed normally of nine segments, and in most butterflies is shorter than the hind wings. On the last segment there are various appendages, which are mainly sexual in their nature.