BOTANICAL GAZETTE. 2S8 



Diptera. On P. tremuhides I observed the following visitors: Aiigo- 

 chlora pura, Selandria Ruhi, Microgaster sp., Phytodiclus vidgaris, 

 Halidus sp. (identified by Mr. Cresson), and numerous small flies 

 which have not been determined, beside a multitude of ants, of which 

 Mr. McCook kindly identified Formica exsectoidcs, F. fi/sca, F. gagatcs, 

 Crematogaster lincolata, and Doryiiiyniicx pyramicus. The common 

 two-spotted CoccincUa was also frequently seen to feed upon the secre- 

 tion of these glands. Many of these insects were also found upon 

 P. graiidideutata and P. i/ionilifera, and in addition I noticed one 

 species of Andrcna upon the former. The first thing that strikes one 

 in studying the insects which feed upon the extrafloral nectar of our 

 ])oplars is their variety: — Coleoptera, Diptera, and both parasitic and 

 imparasitic Hymenoptera are found, the most numerous being ants, 

 and iclineumonid parasites. As is usual in such cases, the ants as a 

 rule show a disposition to fight, rather than give up their places by 

 the glands, over which they sometimes remain for hours ; and some 

 species are so pugnacious that the slighest jar to the branch upon 

 which they are is sufficient to cause them to assume the offensive, and, 

 with mandibles open, they rush about in search of the cause of the 

 disturbance. On presenting my finger to them on such an occasion I 

 have usually found them manifesting a sufficiently strong desire to bite 

 it, but, as when in attendance upon aphides, they are not generally 

 disposed to leave the plant. Whether it be on account of its greater 

 quantity, or because it possesses a more agreeable flavor, I am unable to 

 say, but the honey-dew of aphides is far more attractive to all of the 

 ants observed than the nectar from extrafloral glands, not only in this, 

 but in other genera of plants; and Belt'* has found the same to be true 

 of the honey dew of coccids. In 1880 the poplars about Ithaca, 

 N. Y. , were badly infested with plant-lice which I referred to Chaiio- 

 phorus populicola ;and it was noticeable that after these insects became 

 established on a tree the ants, previously very abundant about the 

 foliar glands, soon transferred their attention to the aphides, so that 

 as a rule the only visitors to the glands were small bees, flies, and 

 Microgasters. Besides the C/iaitophoriis, one other plant-louse was 

 found upon the leaves of P. tremiiloides, but it was not identified. Both 

 these insects were found suffering from the attacks of some parasite, 

 and large numbers were to he seen greatly swollen and either dead 

 or dying. Observation showed that, in the <ase of the C/iaitophonis, 

 one of the Microgasters found about the nectar-glands of the plant vvas 

 the cause of this; and several times one of the parasites, after feasting 

 upon the secretion <)f these glands, was seen 10 begin a search for 

 suitable c.ipdidates for its favor Carefully examining the aphides 

 with its antennae, as it moved through them, the Microgaster rejected 

 all beyond a certain size, knowing that they would have reached 

 maturity before its own offspring had attained its full development ; but 

 when one of the right age was discovered, her ovipositor was quickly 

 brought forward under her body, and a single thrust lodged the egg 



'^ Naturalist in Nicaragua, p. 'i2"). 



