30 DiaspincB. 



The divisions of the head and thorax are ill defined. The 

 abdomen consists of a certain number of free basal segments and 

 a large compound flattened terminal piece known as the pygidium, 

 which is usually triangular in form and more highly chitinised than 

 the rest of the body. Taking fourteen as the typical number of 

 somites in an insect, of which the first four go to make up the 

 head and thorax, we have ten segments allotted to the abdomen 

 But sometimes only two free abdominal segments can be distin- 

 guished above the pygidium of the Diaspina^, and never more than 

 SIX. We have, then, from four to eight segments to be accounted 

 for m.the space occupied by the pygidium. It is difficult to make 

 out the full number of divisions, and probably some of the seg- 

 ments have been reduced to infinitesimal proportions. In some 

 species there are regular series of oval pores and interrupted trans- 

 verse lines indicating the boundaries of suppressed segments, and 

 it IS probable that the various marginal lobes and groups of spine- 

 like processes are each referable to one of these divisions. It is 

 however, as Dr. David Sharp points out {Cambridge Nat. Hist. 

 Insects, Part I. p. 89), at present premature to say that all insects 

 are made up of the same number of primary segments. 



It is in this third and final stage that we find for the first time 

 an external genital orifice, which is usually situated about the 

 middle of the under surface of the pygidium, though in the genus 

 Aonidta It will be found nearer the base of the segment. Around 

 the genital orifice are often disposed several groups of glandular 

 organs whose function is still rather obscure. They have been 

 variously termed by different authors, ' grouped spinnerets,' ' grouped 

 abdominal glands,' 'wax glands,' and ' circumgenital glands,' of 

 which the last term is perhaps the most correct. There are usually 

 five distinct groups (//. I. fig. 14), distinguished as the median {a\ 

 the upper laterals {b), and the lower laterals {c). In some species 

 of Fionma the median and upper laterals are united into a con- 

 tinuous arch. In many species of Aspidiotus there are four groups 

 only, the median being absent. In Poliaspis two concentric series 

 of groups are present, and there are examples in nearly every 

 genus in which the circumgenital glands are entirely wanting. 

 Their absence in many species proves that they are not concerned 

 in the secretion of the puparium. They are situate inside the body 

 towards the ventral side, and communicate with the surface by 

 minute pores which, in the living insect, are usually masked by an 

 efflorescence of white waxy powder. In an article published in 



