"The structure of the veins can be best observed on the under surface of the wing, where they are more prominent. The student should begin by completely denuding of scales a few wings of common species: the wing should be cut off and laid on a moistened piece of glass, to which it will adhere; the scales should then be removed, first from one surface and then from the other, with a fine, moist camel's-hair brush—an operation requiring a little patience and delicacy of touch; the veins will thus be rendered conspicuous.[[2]] When, however, the student has familiarised himself with the general subject, it will not be found necessary in practice to resort to this process; most details will be easily observed without denudation[[3]]; where this is not the case (as where the veins are closely crowded or otherwise obscured), the scales can be removed with the brush on the under surface in the locality of the difficulty only, without cutting off the wing or otherwise damaging the specimen, which remains in the collection available for all purposes as before; with proper practice, even the smallest species are amenable to this treatment, which does not require more skill than the actual setting of the specimen. Some workers prefer to put a drop of benzine on the spot, which renders it temporarily transparent; the effect is short-lived, as the benzine evaporates rapidly, and the cilia (if long) are liable to be damaged by this method."

The Legs consist of the following joints (see Plate [I]., fig. 21): (1) coxa, (2) trochanter, (3) femur, (4) tibia, (5) tarsus, (6) claw. The tarsus normally consists of five joints, but is more or less aborted when the leg is not employed for walking. The spines (SS) on the tibiæ of the several legs vary considerably in size and number. They are often useful to the systematist for purposes of classification.

THE ABDOMEN

consists of nine segments, some of which are often fused together. It contains the various internal organs, of which the most important are those of Digestion and Reproduction. The Digestive System (Plate [I]., fig. 10) consists of the following organs: A, the œsophagus, or throat; C, the sucking stomach; D, the ventriculus or stomach; E, the small intestine; G, the cæcum; H, the colon; K, the biliary vessels; N, the salivary vessels. The function of the sucking stomach is to exhaust the air in the throat and proboscis, and thus to cause the ascent of the fluids into the stomach when the insect is feeding.

III.—ORIGIN OF SPECIES.

The theory of the origin of species as propounded by Darwin may be thus very briefly summarised:—

Variation.—No two organisms are exactly alike; there is always some variation from the parent form, in some cases very slight, in others considerable. (For examples of variation see Plate [VII]., figs. 1 to 9, varieties of Hydriomena deltoidata; Plate [VIII]., figs. 42 to 47, varieties of Epirranthis alectoraria; Plate [IX]., figs. 6 to 14, varieties of Selidosema productata; Plate [X]., figs. 13 to 23, varieties of Azelina gallaria; Plate [X]., figs. 39 to 47, varieties of Declana floccosa.)

Inheritance.—Many of these variations are inherited—a fact demonstrated by our domestic plants and animals, where man has selected and bred from varieties suitable for his purposes, and has thus produced races in which the variation is permanent. Many of the races of domestic animals differ as much from one another as do some distinct species of wild animals.

Struggle for Existence.—All animals and plants produce far more offspring than can possibly survive, thus giving rise to the struggle for existence. For example: The average number of eggs laid by a Lepidopterous insect is certainly over 100, and in many species this number is greatly exceeded. Assuming each female to lay 100 eggs, the progeny from a single pair would amount, after six generations, to over six million individuals.

Natural Selection, or the Survival of the Fittest.—In the struggle for existence which necessarily results from such a great increase of individuals, those variations which favoured the possessors would be preserved, whilst those which did not, would be gradually exterminated. This principle of the preservation of the favourable varieties in the struggle for life is called Natural Selection, or the Survival of the Fittest.