Terms relating to position are unfortunately used by writers on entomology in various, even in opposite senses. Great confusion exists as to the application of such words as base, apex, transverse, longitudinal. We can best explain the way in which the relative positions and directions of parts should be described by reference to Figure 62. The spot 3 represents an imaginary centre, situated between the thorax and abdomen, to which all the parts of the body are supposed to be related. The Insect should always be described as if it were in the position shown in the Figure, and the terms used should not vary as the position is changed. The creature is placed with ventral surface beneath, and with the appendages extended, like the Insect itself, in a horizontal plane. In the Figure the legs are, for clearness, made to radiate, but in the proper position the anterior pair should be approximate in front, and the middle and hind pairs directed backwards under the body. The legs are not to be treated as if they were hanging from the body, though that is the position they frequently actually assume. The right and left sides, and the upper and lower faces (these latter are frequently also spoken of as sides), are still to retain the same nomenclature even when the position of the specimen is reversed. The base of an organ is that margin that is nearest to the ideal centre, the apex that which is most distant. {113}Thus in Fig. 62, where 1 indicates the front tibia, the apex (A) is broader than the base (B); in the antennae the apex is the front part, while in the cerci the apex is the posterior part; in the last abdominal segment (2) the base (B) is in front of the apex (A). The terms longitudinal and transverse should always be used with reference to the two chief axes of the body-surface; longitudinal referring to the axis extending from before backwards, and transverse to that going across, _i.e._ from side to side. {114}CHAPTER IV ARRANGEMENT OF INTERNAL ORGANS–MUSCLES–NERVOUS SYSTEM–GANGLIONIC CHAIN– BRAIN–SENSE-ORGANS–ALIMENTARY CANAL–MALPIGHIAN TUBES–RESPIRATION–TRACHEAL SYSTEM–FUNCTION OF RESPIRATION–BLOOD OR BLOOD-CHYLE–DORSAL VESSEL OR HEART–FAT-BODY–OVARIES–TESTES–PARTHENOGENESIS–GLANDS. The internal anatomy of Insects may be conveniently dealt with under the following heads:—(1) Muscular system; (2) nervous system; (3) alimentary system (under which may be included secretion and excretion, about which in Insects very little is known); (4) respiratory organs; (5) circulatory system; (6) fat-body; (7) reproductive system.