458 MR. HUGH SCOTT OX AX 



These crystals, wliich melt at 92° C, seem to represent tlie entire substance, whicli is 

 practically a pure chemical compound": a mere trace remains in the alcohol from' 

 which they separated. The wax is clearly a different kind to that called coccerine, 

 described by C. Liebermann (Berichte der Deutschen chem. Gesell., Bd. xviii. p. 1975), 

 which melted at 106°. 



These threads are produced by unicellular glands placed in pairs beneath pits in the 

 integument. Each pit (PI. 34. figs. 4, 10 a, 11, 12, 17) is formed by an invagination of 

 the cuticle, but its walls are thinner than the cuticle covering the surface between the 

 pits (PL 34. fig. 17 ). The area of the cross-section of the pit is about the same throughout. 

 On its floor there are two pore-plates lying close together, so that they appear somewhat 

 like a figure of eight (PI. 34. fig. 10). Tiie terra "pore-plate" is not meant to imply 

 the presence of any perforations in the plate, for I have seen no trace of any such. 

 It is used to indicate a specialised portion of the integument lying over a gland, and 

 through which the secretion of that gland passes to the exterior *. If there are no 

 perforations in these plates, the very interesting physiological question arises as to the 

 form in which the secretion is produced by the glandular cells, and the manner in which 

 it traverses the chitinous membrane between it and the exterior. Berlese {I. c.) considers 

 that in all insects the chitinous integument is uninterrupted by any perforations, so 

 that dermal secretions of all kinds must pass through an extremely thin membrane. 

 Each pore-plate has a broad rim, sloping slightly downwards and inwards, like the 

 surface of a funnel, and more strongly chitinised than the surrounding cuticle ; moreover, 

 the rim is transversely striated, and rmder a high magnification a dark dot is often 

 apparent in the centre of each of the strise. The striae do not appear in sections in any 

 way as pores perforating the rim. The outline of each pore-plate is in the form of 

 an oval flattened at one side, the flattened sides of the two ovals being adjacent, but not 

 quite contiguous. The outer margin of the flattened side of the rim has at its central 

 point a small concavity. The two concavities, being opposite to one another, leave a 

 slightly widened space (PI. 34. fig. 10, h), in which there rise, between the two pore- 

 plates, two minute papillae (PL 34. figs. 11 a, 12, 17 i). These appear to be evaginations 

 of the thin cuticle between the rims ; they are in a line at right angles to the long 

 diameter of the pair of pore-plates, and hence when the pair is seen — as it very frequently 

 is — from the side, the two minute papillae appear as a single one (PL 34. fig. 11, a). 

 However, under a high power, and in some transverse sections, the two can be seen, 

 quite distinct from each other. The space within the rim of each pore-plate is closed 

 by a membrane having an indistinctly dotted or mottled appearance. 



These pore-plates are almost invariably in pairs, as described above. In a single case 

 there were seen three of them together, forming a roughly triangular figure. , They vary 

 considerably in size, larger and smaller pairs being interspersed over the dorsal surface 

 of tl\e trunk ; but the two pore-plates of each individual pair are of equal size. They are 

 exceedingly numerous over the whole dorsal surface of the trunk, where they seem to 

 have no definite orientation, their long diameters lying in all directions. They are 

 extremely conspicuous in preparations of specimens that have been treated with caustic 



* A. Berlese, 'Gli Insetti,' vol. i. p. 492 and footnote. 



