[Illustration FIG. 30.--Cross-section of the sucking-tube of the milkweed butterfly, to show the way in which the halves unite to form a central canal (_c_): _tr_, tracheæ, or air-tubes; _n_, nerves; _m_, _m^3_, muscles of one side. (Magnified 125 diameters.) (Burgess.)]
[Illustration FIG. 31.--Longitudinal section of the head of the milkweed butterfly: _cl_, clypeus; _mx_, left maxilla, the right being removed; _mfl_, floor of mouth; _oe_, oesophagus, or gullet; _ov_, mouth-valve; _sd_, salivary duct; _dm_ and _fm_, dorsal and frontal muscles, which open the sac. (Magnified 20 diameters). (Burgess.)]
_The Head._--The head is globular, its breadth generally exceeding its length. The top is called the _vertex_; the anterior portion, corresponding in location to the human face, is called the _front_. Upon the sides of the head are situated the large _compound eyes_, between which are the _antennæ_, or "feelers," as they are sometimes called. Above the mouth is a smooth horny plate, the _clypeus_. The _labrum_, or upper lip, is quite small. On both sides of the mouth are rudimentary _mandibles_, which are microscopic objects. The true suctorial apparatus is formed by the _maxillæ_, which are produced in the form of semi-cylindrical tubes, which, being brought together and interlocking, form a complete tube, which is known as the _proboscis_, and which, when not in use, is curled up spirally, looking like a watch-spring. At the upper end of the proboscis, in the head, is a bulb-like enlargement, in the walls of which are inserted muscles which have their origin on the inner wall of the head. When these muscles contract, the bulb-like cavity is enlarged, a vacuum is produced, and the fluids in the cup of the flower flow up the proboscis and into the bulb. The bulb is also surrounded by muscles, which, when contracting, compress it. The external opening of the tube has a flap, or valve, which, when the bulb is compressed, closes and causes the fluid in it to flow backward into the gullet and the stomach. The arrangement is mechanically not unlike that in a bulb-syringe used by physicians. The process of feeding in the case of the butterfly is a process of pumping honeyed water out of the flowers into the stomach. The length of the proboscis varies; at its base and on either side are placed what are known as the maxillary palpi, which are very small. The lower lip, or _labium_, which is also almost obsolete in the butterflies, has on either side two organs known as the _labial palpi_, which consist of three joints. In the butterflies the labial palpi are generally well developed, though in some genera they are quite small. The antennæ of butterflies are always provided at the extremity with a club-shaped enlargement, and because of this clubbed form of the antennæ the entire group are known as the _Rhopalocera_, the word being compounded from the Greek word (ῥοπαλον), which means a _club_, and the word (_κερας_) which means a _horn_.