OQ [June, 



"But iu five species, namuJj, P. miiiulissimus, j^U'^it^'^^, major, Jlavipes, and 

 breoipennis, individual examples occur vk-ith fully-developed elytra ; these species are 

 therefore dimorphous. The perfect development of the organs of flight has not only 

 a biological but also an important morphological signification. According to the 

 laws of correlation of single organs, the perfect formation of the elytra and wings, for 

 example, is always associated with an essential variation of form of the entire body. 

 The bundles of muscles situate in the thoracic cavity, whose function is to move the 

 wings, are in consequence more massive and stronger, the thorax must therefore be 

 wider and deeper, and the sequence of this is that the entire body is broader and 

 more robust, and instead of being oblong and parallel, approaclies more to an oval 

 form. The pi-onotum, which otherwise is posteriorly only as broad or often still 

 narrower than anteriorly, in this case is broader behind than iu front, — that 

 is, trapezoidal." 



Dr. Horvath says nothing about the probable cause of the occasional alar 

 development in some iTidividuals of a brood. I have on a former occasion (Ent. Mo. 

 Mag. vi, 10, 1869), suggested that such development occurs by a provision of Nature 

 to prevent deterioration of race by breeding " in and in," by enabling some of the 

 "fittest" examples of a species to migrate either to " fresh fields and pastures new," 

 or to mingle with other stocks. But what it is that makes this provision usually 

 latent, or, on the other hand, instigates its exceptional action, is a mystery remaining 

 to be elucidated. Or if it be held that the macropterous form is normal and requires 

 no explanation, the question then is why, as in the genus Plinihisus, the abnormal 

 raicropterous condition exists in the majority of species and individuals ? — 

 J. W. Douglas, Lee : 2Q,th March, 1877. 



On the identity of Trioza ahieticola, Forst. tvith Chermes rhamni, Schrank 

 (in a letter to Mr. Douglas). — After your summary (Ent. Mo. Mag., xiii, p. 255) 

 of all that has hitherto been published concerning those species of PsylUdcB which 

 live upon Rhainnim, and the possibility of any of them being identical with Chermes 

 rhamni, Sclirk., you come to the conclusion that it is rather premature to apply 

 Schrank's specific name " rhamni " to any species, because you are of opinion that 

 absolute certainty is wanting. I say that we have this certainty, and that the insect 

 described by Schrank as Chermes rhamni is without doubt nothing but the larva of 

 Trioza ahieticola, Forst., whereof you may assui'e yourself by observing the larva of 

 this species on the leaves of Mhammis catharticus in the month of May. 



Above all, it is necessary to be convinced that tlie larva described by Schrank 

 can only be that of a Trioza and not of a Psylla, a matter that is easy to be decided 

 by any one who has ever bred insects of these two genera. Schrank says that the 

 whole margin of the body and of the wing-cases of his Chermes rhamni is fringed, 

 ■which is the case with the larvae of the genus Trioza, these fringes being the principal 

 character in which the larvte of this genus differ from those of the genus Psylla, the 

 body of whicli is frequently bordered with hairs but never fringed. Therefore, only 

 the species of Trioza can be taken into consideration, those of Psylla (Ps. rhamni- 

 cola, alaterni, &c.) being in this case quite out of the question. 



Among the Triozce there are only two species feeding upon Rhamnus catharticus, 

 viz., T. Wallccri and T. ahieticola, and of these the larva of the latter only agrees 



