Dear Sir,

In replying to your letter of the 5th instant,¹ I have to remark that immature male spiders usually appear to resemble in colour the females of their respective species of the same age, and that they do not seem to differ materially from them in size till the digital joint of their palpi, by its increasing tumidity, indicates a near approach to maturity, and affords a decided characteristic of sex. Whether these remarks are applicable to the entire order of the Araneidea or not, of course I am incompetent to state; the very remarkable differences in size and colour of the sexes of some species belonging to the genus Nephila may, perhaps, be thought to render it doubtful.

Lycosa lugubris,² Sparassus smaragdulus,³ Philodromus dispar,⁴ Linyphia pratensis⁵ and Linyphia fuliginea⁶ are species the sexes of which resemble each other in colour when young, but differ very decidedly in that respect when adult. I may mention here that young spiders of many species undergo modifications of colour in their progress towards maturity (especially on the occasion of changing their integument), and that such modifications seem to be common to both sexes.

Before your letter came to hand I had collected cocoons of the varieties of Theridion lineatum⁷ for the purpose of observing whether the young on quitting them present any characteristics by which they may be distinguished from each other or not; my present opinion is that they do not, but should the result prove this impression to be inaccurate, I will immediately communicate with you on the subject.

In all my investigations relative to spiders I never heard them produce any sound by means of an organ or organs adapted to that especial purpose; the fact that they do so, if well ascertained, is as you remark highly interesting, and would be still more so should the function be limited to the male sex.

One of my correspondents, the Rev. O. P. Cambridge, has received from Ceylon two species of spiders each of which has only four eyes, descriptions of them will be given in the next number of the ‘Journal of the Linnean Society.’⁸

I was sorry to learn from Miss Holland⁹ that you had met with an accident;¹⁰ hoping that you no longer suffer from the effects of it,

I am, dear Sir, | very truly yours | John Blackwall.