From Francis Walker   9 December 1867

My dear Sir,

I have looked at the Homopterous Insects armed with spines or with horns, & I believe that there is no difference in this respect between male & female. In the Homoptera there is no contrast between the sexes, but only the general difference between one sex & the other.¹ This rule excludes the Aphididæ & the Coccidæ, the latter being most remarkable in the want of resemblance between the sexes. I have, as yet not published on variation in Insects except a few remarks on the question of species in my list of the Diptera of the Eastern Isles in the Journ Linn. Soc—² I am about to publish some notes on the Aphididæ—a translation of Passerini’s synopsis of the species & remarks on the nomenclature, & I have introduced here & there some words on the mutability of species.³ I will write to you all that may appear from me on the latter subject. I have made only one experiment on Aphides. The Aphis of the holly & the Aphis of the ivy seem to be exactly alike tho’ they have been described as two species. I took some shoots of ivy infested by Aphides & tied them with thread to shoots of holly, but tho’ the Aphides wandered to the holly they did not feed on it & soon perished. Probably the Aphis requires the same kind of food from its birth for tho’ some species migrate to a great variety of plants they hardly feed after they have migrated & pass out of existence when they have brought forth a new generation⁴   I believe that the migrations initiate yearly many species (so called) which have no continuity, but pass away before winter. I am just now making out a list of the Blattidæ or Cockroaches— this tribe is remarkable in four particulars— 1^st.—the great difference in some species between male & female   2^nd the incomplete metamorphose of some species or their perpetuity in the apparently larva-state   3^rd. the facility with which some species accomodate themselves to artificial circumstances & thus become cosmopolitan—& 4^th. the disguises which they put on or their mimicry of the Coleoptera, Myriapoda & terrestrial Crustacea with which they are associated in their natural habitations— They thus seem to epitomize some tribes that are inferior to them in structure & to indicate others of higher organization—⁵ Some insects are said to photograph the colours near them— those that are disguised may be said to morphograph other tribes with which they dwell or to be examples of morphography.⁶

I enclose my Carte de visite— may I ask for the favor of yours?—⁷

With my best wishes, | Yours truly | F. Walker

P.S. Your Carte de Visite has just arrived— many thanks for it.⁸