From H. W. Bates   21 February 1868

My dear Mr Darwin

Mr Westwood very unfortunately tried to introduce names for the families of Leaping Orthoptera, different from those used by several authors of Monographs.¹

His

You might use English names for first & last—“Crickets” & “Grass hoppers”—but the second have no English name.²

In “Amazons vol I. 250; I aimed at showing there was a gradation of perfection in the stridulating organs at base of wing-cases.³ What I stated about Crickets was the result of visits to Baker’s ovens at the time,⁴ dissection of fresh specimens & examination of many foreign achetidæ; and on looking at a few species I have at hand I still say that the edges of base of wing cases of Achetidæ are not forced out of symmetry; i.e not produced into lobes of different shapes & widely different structure,—as they are in Locustidæ, where also the organs advance gradually from genus to genus (speaking generally) to a high degree of elaboration.

On looking at Westwood’s figures of Achetidæ, quoted in your letter, I find he does not figure two wing-cases of a male but one of a male & one of a female.⁵

It is however probable that in some true crickets (achetidæ) there may be some differences in the two wing-cases; it would be advisable therefore to avoid making too sweeping an assertion about their symmetry throughout the family;⁶ this is not essential to the argument, which is that there is a gradation in asymmetry & a step by step advance to a wonderful musical instrument.

Want of symmetry in the same pair of wings of insects is an exceedingly rare phenomenon and how it could first occur by variation in these orthoptera, so as to give Nature the first chance of a musical instrument, is a curious question. I think it occurred through the overlapping of the inner edges of base of wing-cases, caused simply by the shape of the wings & body, without any reference to a musical instrument being afterwards elaborated.

Yours sincerely | H W Bates