610 CLASS IV. INSECTA. 



exception) during embryonic life. It corresponds with the 

 second antennary segment of the Crustacea. In many cases, 

 moreover, the abdomen does not form its full complement of 

 segments, and, when it does, some of the hinder ones are 

 pushed or telescoped into those in front and there is usually 

 a certain suppression or atrophy of parts which tends to obscure 

 the real number of segments. 



The abdominal segments primitively bear paired appendages 

 homologous with the legs, but these usually disappear. In 

 the Collembola and Thysanura certain of these appendages persist 

 and some authorities regard the processes which form the 

 ovipositor in some Insects, as appendages and also the anal 

 styles of the male cockroach. The cerci anales (cercopods) are 

 with more certainty regarded as appendages. 



The cuticle which covers all parts of the body of an insect 

 embryo and which is infolded at the several apertures leading 

 into the body, thickens in certain parts to form plates or sclerites. 

 In a typical segment there are the following sclerites : (i) the 

 tergum or dorsal shield, each tergum consists originally of a right 

 and a left half which unite in the middle line in the adult ; (ii) the 

 sternum or ventral plate which may remain double. Between 

 the terga and the sterna is usually a softer membrane, the pleuron, 

 and in this may be found : (iii)an epimeron adjacent to the tergum, 

 and (iv) an episternum next the sternum. In nature however 

 all these four parts are seldom met with together, some may be 

 suppressed, others greatly enlarged, so that the analysis of the 

 exoskeleton of an insect's body, especially of the thorax, is a 

 matter of no small difficulty. 



The head of an Insect consists of a rounded chitinous case in 

 which, except for the appendages, almost all traces of segmenta- 

 tion have disappeared. As a rule the following regions can 

 be made out (Fig. 365). The dypeus is a somewhat flattened 

 area between the bases of the two antennae extending ventral- 

 wards to bear the labrum, which is a hinged flap overhanging 

 the mouth. The labrum is sometimes lobed but is primitively 

 a median process. The top of the head or epicranium runs 

 from the dorsal limit of the clypeus often marked by a 

 suture between the eyes on to the back of the head down 

 as far as the neck. The region of the head surrounding the 

 insertion of the neck is often referred to as the occiput, 



