3. The Proboscis, or Haustellum[[1]] (c), a tubular extensible organ formed of the two maxillæ, or lower jaws, which have become greatly elongated, semi-tubular, and closely pressed together at the edges, but separable at the will of the insect—a structure which enables the organ to be easily cleansed when necessary, and is extremely interesting as indicating so clearly the true development of the proboscis from the maxillæ.
The Maxillary palpi (p.p) are two jointed organs attached to the base of the proboscis and very frequently rudimentary, but fully developed amongst certain of the Micro-Lepidoptera.
The Labium, or lower lip, is situated below the proboscis and carries the Labial palpi (figs. 5 and 6), two large jointed organs which are very conspicuous in nearly all the species and often quite conceal the maxillary palpi. They are usually regarded as organs of touch, but their true function does not seem to be properly understood. In the Lepidoptera they appear to protect the proboscis, which, when out of use, is always coiled up in a spiral between them. The labrum and mandibles can only be seen by removing the large labial palpi.
THE THORAX
carries the organs of locomotion, which consist of two pairs of wings attached to its sides, and three pairs of legs attached beneath, a pair belonging to each of the three segments of which the thorax is composed. On the front of the thorax there are two flap-like organs covered with scales, termed the patagia.
The Wings vary greatly in shape, but usually they are triangular. The portion of the wing which joins on to the thorax is termed the base. The front margin is called the costa, the outer margin the termen, and the lower margin the dorsum, these being described as situated when the wing is extended in flight. The angle between the costa and termen is called the apex, and the angle between the termen and the dorsum the tornus (see Plate [I]., fig. 1). The termen and dorsum are edged with a fringe of hair-like scales, termed the cilia. At the base of the hind-wings is generally situated a stiff bristle, or several stiff hairs, called the frenulum, the ends of which pass through a chitinous process on the under side of the fore-wing near the dorsum. This process is termed the retinaculum, and serves, in conjunction with the frenulum, to lock the wings together during flight. In the female both these organs are often very imperfectly developed, the frenulum consisting of several bristly hairs, and the retinaculum of a group of stiff scales. In many of the Lepidoptera both frenulum and retinaculum are entirely wanting.
"In the Micropterygina, a membranous or spine-like process called the jugum rises from the dorsum of the fore-wing near the base and passes under the hind-wing, which is thus held between the process and the overlapping portion of the fore-wing."—(Meyrick.)
The veins of the wings are thus described by Mr. Meyrick:—
"The wings are traversed by a system of Veins—tubular structures which serve at once as extensions of the tracheal system, and to form a stiff framework for the support of the wing. In the normal type of Lepidoptera the fore-wings possess three free veins towards the dorsum, termed 1a, 1b, and 1c; a central cell, out of which rise ten veins, numbered 2 to 11, the sides of the cell being known as the upper median, lower median, and transverse veins respectively; and a free subcostal vein, numbered 12; whilst the hind-wings differ from the fore-wings in having only six veins rising from the central cell, numbered 2 to 7, so that the free subcostal vein is numbered 8 (see Plate [I]., figs. 3 and 4, assumed type of neuration of a Lepidopterous insect). In some forms a forked parting-vein traverses the middle of the cell longitudinally, and a second parting-vein traverses the upper portion, so as to form a secondary cell; but these are more frequently absent or represented only by folds in the membrane. In a few forms there is a tendency to the production of several false veins, termed pseudoneuria, appearing as short branches from the subcostal vein of the hind-wings to the costa; these are thickenings of the membrane, and are commonly very irregular and variable, often uneven in thickness or incomplete. Sometimes one of these near the base is better developed and more permanent in character; it is then termed the præcostal spur (see Plate [I]., figs. 89 and 279). Modifications in the general arrangement of the veins may arise through any of the following processes, viz.: (1) obsolescence, when a vein loses its normal tubular structure, becoming attenuated and reduced in substance, until it appears a mere fold of the membrane (Plate [II]., fig. 60, vein 5 in hind-wings of Selidosema); (2) stalking, when the two veins are fused together for a portion of their length from their base, so as to appear to rise on a common stalk (Plate [II]., fig. 34, veins 6 and 7 in hind-wing of Hydriomena); (3) coincidence, when two veins are fused together for the whole of their length, so that one appears entirely absent, an extreme form of stalking; (4) anastomosis, when two veins rise separate, meet, and are fused together for a certain distance, and then separate again (Plate [II]., fig. 23, veins 7 and 8 in the hind-wings of the ♀ of Tatosoma); (5) concurrence, when a vein rises separate, runs into another, and does not separate again, an extreme form of anastomosis; (6) connection, when two veins are connected by a short transverse bar passing from one to the other, a special form of anastomosis, evolved from the ordinary form under the influence of a tendency to lateral extension (Plate [II]., fig. 28, veins 7 and 8 in hind-wing of Paradetis). Vein 1b in both wings is often furcate at the base.
"The type of veins in the Micropterygina differs from that described above in two essential particulars, viz.: (1) there may be three additional veins in the fore-wings, rising out of vein 11 or 12; and (2) the veins of the hind-wings are practically identical in number and structure with those of the fore-wings, being thus much more numerous than in the ordinary type. There is also often a system of cross-bars between the veins near the base of the wing (Plate [I]., figs. 22 and 23, neuration of Hepialus).