To J. D. Hooker   16 July 1874

My dear Hooker

Surely the acacia must be the veritable “Bull’s-horns” described by Belt, with the curious little tips which secrete nectar & nourish the protecting ants.¹ If you have the plant alive some of you botanists ought to examine the development structure & secretion of the little tips. By the way Fritz Müller tells me (to whom I sent Belt’s book) that he is certain that some Cecropias cannot possibly there exist without the protection of ants.²

I am rather glad you have not been able to send Utricularia, for the common species has driven Frank & me almost mad.³ The structure is most complex. The bladders catch a multitude of Entomostraca⁴ & larvæ of insects. The mechanism for capture is excellent. But there is much that we cannot understand. From what I have seen today I strongly suspect that it is necrophogous i.e. that it cannot digest but absorbs decaying animal matter.

Foster is certainly in error about the aggregation of the protoplasm: every insect which Drosera catches causes aggregation, & the aggregated matter afterwards redissolves.⁵ Acids which are poisonous do not cause true aggregation. If I remember right citric acid is innocuous.

Many thanks, I am much better but I had a bad attack on Monday—⁶

Yours affectly | Ch. Darwin