Additional pieces may be developed between the terga and sterna, and these have long been termed _pleural_.[38] There may be, for example, single stigmatic plates, as in the abdomen of the Cockroach, pieces to support the thoracic legs, and pieces to support the wings; but the number and position of these plates depends upon their immediate use, and their homologies become very uncertain when Insects of different orders are compared. In general, the pleural elements of the segment are late in development, variable, and highly adaptive. [38] This application of the word to denote parts intermediate between terga and sterna has become general since its adoption by Audouin. It appears also in the older and deservedly obsolete nomenclature of Kirby and Spence. Professor Huxley has unfortunately disturbed the consistent use of this term by giving the name _pleura_ to the free edges of the terga in Crustacea. _Somites of the Cockroach._ The exoskeleton of the Cockroach is divisible into about seventeen segments, which are grouped into three regions, as follows:-- Head {Procephalic lobes {Post-oral segments 3 Thorax 3[39] Abdomen 11 -- 17 -- [39] Where the thorax apparently consists of four somites, as in some Hymenoptera, Hemiptera, Coleoptera, and Lepidoptera, the first abdominal segment has become blended with it. It is a strong argument in favour of this estimate that many Insects, at the time when segmentation first appears, possess seventeen segments.[40] The procephalic lobes, from which a great part of the head, including the antennæ, is developed, are often counted as an additional segment.[41] [40] Balfour. Embryology, Vol. I., p. 337. [41] _E.g._, by Graber. Insekten, Vol. II., p. 423. The limbs, which in less specialised Arthropoda are carried with great regularity on every segment of the body, are greatly reduced in Insects. Those borne by the head are converted into sensory and masticatory organs; those on the abdomen are either totally suppressed, or extremely modified, and only the thoracic limbs remain capable of aiding in locomotion. The primitive structure of the Arthropod limb is adapted to locomotion in water, and persists, with little modification, in most Crustacea. Here we find in most of the appendages[42] a basal stalk (_protopodite_), often two-jointed, an inner terminal branch (_endopodite_), and an outer terminal branch (_exopodite_), each of the latter commonly consisting of several joints. It does not appear that the appendages of Insects conform to the biramous Crustacean type, though the ends of the maxillæ are often divided into an outer and an inner portion. [42] See, for example, Huxley on the Crayfish.