3 2 DiaspincE. 



the middle of the segment. In doubtful cases, this character may 

 perhaps assist in the determination of the genus. 



The margin of the pygidium bears various processes that are 

 found of great use in the differentiation of species. They are 

 almost constant in the same species, while showing a great variation 

 between distinct species. The most conspicuous of these processes 

 are the prominent tooth-like chitinous lobes {pi. I. fig. 14, i) of 

 which there may be from one to four pairs. The central pair 

 sometimes coalesce to form a single median lobe. The lobes, 

 together with the simple marginal prominences (from which it is 

 sometimes difficult to separate them) seem to be employed in 

 shifting obstacles, such as hairs and fibrous matter on the surface 

 of the plant, that would otherwise interfere with the formation of 

 the puparium. They are often strengthened by thickened in- 

 growths of the body-wall. 



Between and beyond the lobes are the ' squames ' (' plates ' of 

 Comstock, ' scaly hairs ' of Maskell), which may be flattened and 

 fimbriated as in Parlatoria and many species of Aspidiotiis 

 {fig. 16, d, e,fi) or tapering and spiniform as in Diaspis and Mytil- 

 aspis {fig. 16, a), with numerous intermediate forms. The squames 

 are concerned in the weaving of the scale and are associated with 

 some of the tubular spinnerets described below. 



There are also a few inconspicuous simple 'spines' {fig. 14, ;«) 

 occurring usually in pairs, of which one is placed dorsally and the 

 other ventrally. Each pair of these spines probably indicates a 

 suppressed segment of the body. 



The 'tubular spinnerets' are perhaps the most important organs 

 connected with the pygidium, though they are far from being the 

 most conspicuous. They are the principal if not the sole agents 

 concerned in the secretion of the covering-scale. They are not 

 confined to the pygidium, but occur in many species on the free 

 abdominal segments also, either in groups on the lateral margin 

 or extending in a transverse row across the segments. There are 

 two principal forms noticeable : the ' cylindrical ' which consists 

 of a comparatively short and broad tube with parallel sides 

 {fig. 14, /i), and the ' fihform ' which extends far into the body in 

 the form of a narrow thread-like tube {fig. 14, g). Between these 

 extremes maybe distinguished a 'trumpet-shaped' tube {fig. 14,/), 

 but these three types may be connected by many intermediate 

 forms. They are all constructed on a similar plan. At the free 

 inner extremity of the tube is a capitate organ, with a thickened 



