The lower or less-specialised Insects, such as the Cockroach, have nearly as many nerve-ganglia as segments, and the longitudinal connectives of the nerve-cord are double. In the adult of certain higher Insects[9] (_e.g._, many Coleoptera, and some Diptera) the nerve-ganglia are concentrated, reduced in number, and restricted to the head and thorax; while all the connectives, except those of the œsophageal ring, may be outwardly single. [9] So also in some larvæ (_Calandra_, _Œstrus_, &c.). The heart, or dorsal vessel, is subdivided by constrictions into a series of chambers, from which an aorta passes forwards to the head. Air is usually taken into the body by stigmata or breathing-pores,[10] which lie along the sides of the thorax and abdomen. It circulates through repeatedly-branching tracheal tubes, whose lining is strengthened by a spiral coil. Air-sacs (dilated portions of the air-tubes) occur in Insects of powerful flight. [10] In some aquatic Insects the exchange of gases is effected by “pseudobranchiæ,” and the tracheal system is closed. The generative organs are placed near the hinder end of the body.[11] Most Insects are oviparous.[12] The sexes are always distinct; but imperfect females (“neuters”) occur in some kinds of social Insects. Agamogenesis (reproduction by unfertilised eggs) is not uncommon. [11] Dragon-flies have the male copulatory apparatus, but not the genital aperture, in the fore part of the abdomen. [12] Aphis and Cecidomyia are at times viviparous, and a viviparous Moth has been observed by Fritz Müller (Trans. Entom. Soc. Lond., 1883). _Orders of Insects._ The orders of Insects are usually defined with reference to the degree of metamorphosis and the structure of the parts of the mouth. Five of the orders (3, 5–8) in the table on page 9 undergo complete metamorphosis, and during the time of most rapid change the insect is motionless. In the remaining orders (1, 2, 4) there is either no metamorphosis (_Thysanura_), or it is incomplete--_i.e._, the insect is active in all stages of growth. Among these three orders we readily distinguish the minute and wingless Thysanura. Two orders remain, in which the adult is commonly provided with wings; of these, the Orthoptera have biting jaws, the Hemiptera, jaws adapted for piercing and sucking. The name of Black Beetle, often given to the Cockroach, is therefore technically wrong. True Beetles have a resting or chrysalis stage, and may further be recognised in the adult state by the dense wing-covers, meeting along a straight line down the middle of the back, and by the transversely folded wings. Cockroaches have no resting stage, the wing-covers overlap, and the wings fold up fan-wise. _Further Definition of Cockroaches._ In the large order of Orthoptera, which includes Earwigs, Praying Insects, Walking Sticks, Grasshoppers, Locusts, Crickets, White Ants, Day-flies, and Dragon-flies, the family of Cockroaches is defined as follows:--