My dear Darwin,

You have afforded me much gratification by sending me your new little work on Forms of Flowers.¹ I am reading it with much interest, and with the thought of how much patient research and deep pondering it represents.

I was glad to find that the little fig-parasites interested you, and I have for some time been going to write to you further respecting them.² They still occupy some of my spare time, and I have been sending from time to time consignments of these little insects to our good friend Westwood, who has written to tell me how greatly pleased he is with them; but he is rather too sparing of his letters to me about them—³

About a dozen species of figs have already yielded more than 30 kinds of which the males have no wings or very rudimentary ones; and I am still finding more. There are also to be added several of the same tribe of insects but with both sexes winged.

Each kind of fig is infested with from 2 to 7 or 8 commensal species, and the individual insects are frequently so very numerous that every seed-vessel must have been occupied; and one seed is often food for 2 insects which before maturity can be seen curiously packed together in the seed vessel.

The genera are not numerous, but are mostly represented by distinct species occurring in the several kinds of fig, as it seems to me, for I am only certain having found the same species of insect in two kinds of fig, though Westwood may possibly have detected other instances.⁴ I have not had time to examine these tiny insects so critically as I could wish.

At one time of the year the same species of these parasites are found in the fruits of the same kinds of fig respectively; but I am finding other species in some of the same fruits ripening some time subsequently, but to what extent this may go remains to be seen.

A bletting of the outer portion of the fig-fruit would seem to be the result in some cases of the presence of the parasites, but it is by no means general, even in the one species where I have observed it to be present.⁵ There is usually nothing about the fruit externally to indicate the presence of parasites   In the very young fruits their interior is mostly quite filled with the crowded flowers, but towards maturity a cavity is found in the centre to which the tips of the seed vessels just reach, and into this cavity first emerge the little insects, copulate there, and then the females escape from the apical aperture formed either naturally or by the action of the powerful mandibles of the males. The males of some of the species run about very actively, and fight fiercely with kindred males. Males of other species are very sluggish in their movements.

The insects are best collected some little time before the fruit is ripe. Only a few of the males & these dead are to be found in the fallen fruits.

With kind regards I am always most truly your’s | G. H. K. Thwaites