432 SUMMARY OF CURRENT RESEARCHES RELATING TO 



are not secreted by processes but by gland-cells. Eather thick hairs, 

 found on the peripheral portion of the dermal region, on the rudiments 

 of the wings, and most numerously at the hinder end of the body, are 

 seen in Psyllopsis to be of two kinds ; some have a wide lumen and 

 thin walls, the other a much narrower lumen and very thick walls ; 

 they differ in form in various genera. The tracheal system is difficult 

 to make out in the imagines, but much easier in the larvse ; in the 

 latter there are nine pairs of stigmata ; these lead into short trunks 

 which are not much thicker than the branches, into which they soon 

 break up ; their distribution is described in detail. As in the Aphides, 

 the tubes consist of a layer of fused cells, which secrete the spirally 

 thickened chitinous cuticle. The apparatus for closing the stigmata 

 is described, and the account does not correspond with that given by 

 Landois ; the author thinks that we have to do with an arrangement 

 which is involuntary. 



The external structure of the nervous system of the Psyllidse 

 resembles that of the Aphides, and consists of the same parts as in 

 other insects ; internally are the fibrous masses which follow a very 

 complicated course, but finally terminate at one end in a sensory organ 

 or a peripheral nerve, and at the other in the cortical layer, in the 

 cells of which they end. The author was unable to find the masses of 

 cells in the interior of the brain, which have been described by Berger. 

 He agrees with Michels in denying the existence of a dotted substance, 

 the appearance of which is ascribed to the fibres which are cut through 

 in sections. The digestive apparatus is somewhat more complicated 

 than in the closely allied Aphides, forming loops as in the Coccid^e 

 and some Cicadidfe ; the suctorial apparatus, however, is exactly on 

 the type of that of the Aphides. The dorsal vessel, and the pseudo- 

 vitellus (secondary yolk of Metschnikoff) are as in Aphides ; but the 

 latter is of a brown colour. After describing the generative apparatus, 

 to which in many points the same remark applies, the author concludes 

 by discussing the genetic relationships of the Psyllidse. 



By their internal characters, as well as by the fact that the male 

 and female differ little externally, the Psyllidse stand nearest to the 

 Cicadellidse ; the Aphides may perhaps be similarly referred ; but 

 certain forms have become markedly adapted to their parasitic mode 

 of life, and especially the apterous generations which live in galls. 

 The most primitive types must be those winged parthenogenetic 

 females which most resemble the males, which most closely resemble 

 one another in the different species, and which approach the 

 males in the form of the body, wings, accessory eyes, antennae, &c. 

 The Chermetidse present similar relations to the gall-dwelling 

 Aphides ; here we have winged parthenogenetic females which are 

 like those of the Aphides, but no winged males ; the last are either 

 apparently wanting, as in Ghermes dbietis, or, as in Phylloxera quercus, 

 and P. vastatrix, as also in many Pemphigin^, they are, like their 

 proper females, quite small and apterous. In some anatomical points 

 the Chermetidse appear to form a passage from the Aphides to the 

 Coccidse ; the latter differ so much from the rest of the Phytophthires 

 that we must perhaps suppose that they had a special origin from the 



