viii INTRODUCTIOyr. 



hence their net-Hke appearance. The colour varies; it may be grey, brown, red, green, black, 

 etc. The accessory eyes, ocelli or simple eyes as they are sometimes called, are placed on 

 the vertex, and are generally hidden under the scales. They do not occur in butterflies. 



The thorax is composed of three horny segments fused together, with only indistinct 

 sutures between them. The first thoracic segment, the prothorax, is closely united to the 

 second and is narrow on the upper side (the pronotum) but wider beneath (the prosternum); 

 where there are two grooves for the articulation of the first pair of legs. The second thoracic 

 segment, the mesothorax, whose upper part is called the mesonotum and has a small projection 

 behind (the scutellum), is grooved on the sides where the fore wings are attached, the base of 

 which is covered by two small plates called the tegulae. The lower part is called the meso- 

 sternum. The third segment, the metathorax, is narrower than the second, and is called the 

 metanotum above, and the metasternum below. It has the hind wings on the sides, and the 

 third pair of legs beneath. The under side of the thorax is termed collectively the pectus. 



The abdomen is formed of eight or nine segments and varies considerably in size. It 

 is sometimes shorter than the hind wings, but frequently projects beyond them. There are 

 occasionally erect hairs on the back and often similar tufts on the sides of the extremity of 

 the abdomen, which are called the anal tuft. On the last segment there are various appen- 

 dages in many species. There is occasionally an ovipositor in the female. 



The wings are membranous leaf-like expansions, which are crossed by horny tubes, 

 called nervures or veins, and are covered on both surfaces with small scales either uniformly 

 distributed or unequally dispersed. The nervures really consist of two tubes, one within the 

 other. The outer of these is filled with blood, and the inner with air. Thus the process of 

 aeration of the blood is carried on in the wings. The wings are attached by a joint, the fore 

 wings to the mesothorax and the hind wings to the metathorax. The wings are more or less 

 triangular in shape, with three sides or edges, which, when they are spread out, are distin- 

 guished as the front margin or costa, the inner margin, and the outer or hind margin. The 

 angle between the costa and the hind margin is called the apex or tip, and that between 

 the inner and hind margins the hinder angle in the fore wings, and the anal angle in the hind 

 wings. The place where the wings join with the body is called the base. The form of the 

 wings depends upon the shape of the angles and margins, and is described as oval, rounded, 

 falcate, etc. The margins of the wings may be straight, curved, scalloped, undulated, or dentated. 

 On the hind margins there is usually a marginal line before the base of the fringes. The 

 fringes consist of hair-like scales placed on the free edge, and vary much in length and colour. 

 In some species the hind wings have long narrow appendages called tails; in others, as in the 

 plume moths, the w'ings are split up into several feather-like portions nearly to the base. The 

 position of the wings when at rest differs in different groups. They are sometimes held erect 

 with their upper surfaces in contact, extended horizontally, in a position midway between the 

 two, or pressed to the sides of the body. The fore wings may completely cover the hind wings 

 when at rest, or the hind wings may project over the fore wings. In the females of some of 

 the Bomhyces the wings are rudimentary or entirely wanting. 



The shape of the wings primarily depends upon the nervures. These rise from several 

 main stems at the base of the wings, and run to the margins, some of them singly, others 

 branching. They divide the wings mto a number of spaces w-hich are called cells. The 

 nervure which runs nearest to the costa is called the costal nervure. It is thickest on the 

 fore wings. The two next which rise from the middle of the base of the wings, and are 

 conspicuous for their size and shape, are called respectively the subcostal and median nervures, 

 and enclose a large space between them called the discoidal cell. They branch several times 

 and are generally united at the extremity of the cell by the disco-cellular nervules. If these 

 branches are wanting, the discoidal cell is said to be open. Below the median nervure from 

 one to three slender nervures rise from the base and run to the margins without branching; 

 these are called submcdian or internal nervures. 



