Dear Sir,

I wish it were in my power to give satisfactory answers to the questions contained in your letter of the 10th instant.¹ Adult spiders of the same species differ so remarkably in size and colour, and that independently, to all appearance, of the situations in which they are found, that I am unable to assign an adequate cause for this extraordinary fact. At one time I was disposed to suspect that a deficiency of light might influence the colour of such species as habitually pass their lives in obscure gloomy places; but I have observed such marked differences in individuals of the same species inhabiting the same or similar localities that I refrain from expressing a decided opinion on the subject. The several striking varieties of Theridion lineatum to which you allude, I have frequently met with in close proximity, and that they are specifically identical cannot be doubted, as they entirely coincide in their external structure, habits and economy.² Foreign spiders seem to present variations in colour as conspicuous as those observed in our indigenous species, but it commonly happens that very little is known of their habits and haunts.

In numerous instances the sexes of spiders differ greatly in size and colour, and the males in some cases bear a very small proportion, apparently, to the females in point of numbers.³ Among the spiders provided with spines on the abdomen, belonging to the genus Gasteracantha, I have not yet seen a male; and Walckenaer and Koch⁴ do not appear to have been more fortunate than myself in that respect; the only descriptions of this sex with which I am acquainted are very brief ones of two species given by Thorell, the Swedish arachnologist, from which I infer that the abdominal spines do not constitute a sexual characteristic.⁵

The courtship of spiders is usually a very prolonged and tedious affair; I have frequently observed two or more males on the same web with a female, but I do not recollect having seen them fight for the possession of her; they are generally extremely cautious in making their advances.⁶

Spiders are subject to frequent variations in the number and size of their eyes, a circumstance of much importance when considered in relation to their systematic arrangement. In the spring of last year I received an undescribed species of Pholcus⁷ from India, in which the two sexes were united, the left side being that of an adult male, and the right side that of a female; this must be a case of exceedingly rare occurrence, as it is the only instance of the kind that has come under my observation, and I have not met with any notice of a similar one in the course of my reading.

Regretting that the information in my possession relative to the particulars which form the subjects of your inquiry is so very scanty, | I am, dear Sir, | very truly yours | John Blackwall.