The date is established by the reference to the first part of Lubbock 1863–5 (see n. 2, below).

CD refers to the first part of Lubbock’s paper ‘On the development of Chloeon (Ephemera) dimidiatum’ (Lubbock 1863–5), in which he identified over twenty stages through which the larvae of the pond insect Chloëon dimidiatum, one of the Ephemeridae, pass before reaching maturity. Lubbock read the first part of the paper before the Linnean Society on 15 January 1863, and may have requested CD’s comments on it when he dined at Down House on 22 February 1863 (see n. 8, below). There are two lightly annotated copies of Lubbock 1863–5 in the Darwin Pamphlet Collection–CUL.

In his introductory remarks (Lubbock 1863–5, pp. 61–6), Lubbock noted that entomologists believed it a general rule that every insect had four distinct periods of existence, each marked by an alteration of form, namely, the egg, the larva, the pupa, and the imago. Lubbock, however, drew attention to the fact that in several insects there was no such well-marked four-stage metamorphosis, and that not all insects left the egg in the same stage of ‘embryonal development’. Lubbock argued that the condition of an insect at birth depended ‘partly on the group to which it belongs, but perhaps still more on the manner in which it is to live’ (ibid., p. 63). Lubbock pointed out that several exceptions to the entomologists’ simple model of four stages of development had already been established, including Meloë and Sitaris among the Coleoptera, Lonchoptera and ‘Pupipara’ (formerly a sub-order comprising the families Hippoboscidae and Nycteribiidae) among the Diptera, Typhlocyba and Aphis among the Hemiptera, Psocus among the Neuroptera, Thrips among the Thysanoptera, and Gryllus campestris among the Orthoptera.

CD refers to Lubbock’s point regarding the genera Meloë and Sitaris (Lubbock 1863–5, pp. 63–4), which begin life as active hexapod larvae, but having introduced themselves into the cells of certain species of Hymenoptera, they undergo a retrograde metamorphosis, lose their legs, and emerge as grubs, not altogether unlike those whose places they have usurped.

In his copies of the first part of Lubbock 1863–5, CD annotated passages dealing with the connections between insect form during different stages of metamorphosis, and the insect’s food supply and habits. Lubbock argued that both the speed at which different organs develop, and the form the metamorphosis took, depended upon adaptation to external conditions (ibid., pp. 63–4); he developed this argument further in Lubbock 1866. In the fourth edition of Origin, CD cited Lubbock’s studies of insect metamorphosis as examples of gradual developmental transformation, and of how embryos could be related to their ‘conditions of existence’ (Origin 4th ed., pp. 517, 520).

CD refers to Lubbock’s remarks on the relative lack of freshwater fauna, compared with the variety of marine fauna (Lubbock 1863–5, pp. 64–5): The Mollusca are far less numerous and less varied; of the Fish the same may be said; compared with those of the sea, our freshwater Bryozoa are quite insignificant in numbers; the Hydrozoa are represented by only two genera. Among Crustacea, the Podophthalms have in this country but one freshwater species, the Isopods one, the Amphipods very few; Entomostraca, indeed, are well represented, but Cirrhipedes are altogether absent; neither the Actinozoa nor the Echinodermata have a single freshwater representative.

CD had previously encouraged Daniel Oliver to undertake a study of freshwater plants (see letter to Daniel Oliver, 20 [January 1863], and Correspondence vol. 10, letter to Daniel Oliver, 12 [April 1862]).

Emma Darwin recorded in her diary (DAR 242) that John Lubbock dined at Down House on 22 February 1863. See also letter from John Lubbock, 20 February 1863.