The date is established by the content of the letter (see n. 2, below).

Between April and June 1863, CD was observing what insects visited the orchid genus Cypripedium and trying to determine how fertilisation was effected (see Correspondence vol. 18, Supplement, letter to Arthur Rawson, 6 June [1863], and CD’s observational notes on C. pubescens and C. acaule, dated April and May 1863, in DAR 70: 112–13 and 121; see also Correspondence vol. 11, letter from Asa Gray, 26 May 1863). In Orchids, pp. 274–5, CD had speculated on the means by which flowers of Cypripedium might be fertilised, concluding that an insect would have to insert its proboscis through one of two small openings above the lateral anthers. Asa Gray concluded, from observations of American species of Cypripedium, that an insect would enter a flower through the large opening on the dorsal surface of the flower, then crawl out through one of the small openings above the anthers, which would become visible to them after feeding on the nectar directly underneath the stigma (see Correspondence vol. 10, letter to Asa Gray, 10–20 June [1862] and n. 16, and A. Gray 1862b, p. 428). Hymenoptera is the large order of insects that includes sawflies, wasps, bees and ants. N. of salver: presumably nitrate of silver, although that would not be precipitated by an acid. The word saltica had been changed to salver in a contemporary pencil annotation.