Fritz Müller has given a brief account, under the name of the Imbauba ant, of a Brazilian arboreal ant, that forms small nests in the interior of plants. The species referred to is no doubt an _Azteca_, and either _A. instabilis_, or _A. mulleri_. The nests are founded by fertilised females which may frequently be found in the cells on young _Cecropia_ plants. Each internode, he says, has on the outside, near its upper part, a small pit where the wall is much thinner, and in this the female makes a hole by which she enters. Soon afterwards the hole is completely closed by a luxuriant excrescence from its margins, and it remains thus closed until about a dozen workers have developed from the eggs of the female, when the hole is opened anew from within by the workers. It is said that many of the larvae of these ants are devoured by the grubs of a parasite of the family Chalcididae. This Insect is thought to protect the plant from the attacks of leaf-cutting ants of the genus _Atta_. We may here briefly remark that much has been written about the benefits conferred on plants by the protection given to them in various ways by ants: but there is reason to suppose that a critical view of the subject will not support the idea of the association being of supreme importance to the trees.[66] SUB-FAM. 3. MYRMICIDES.—_Pedicel of abdomen formed of two well-marked nodes (knot-like segments). Sting present (absent in the Cryptocerini and Attini). (It should be noted that the {159}workers of the genera Eciton and Aenictus of the subfamily Dorylides have, like the Myrmicides, two nodes in the pedicel.)_ This sub-family consists of about 1000 species, and includes a great variety of forms, but, as they are most of them of small size, they are less known than the Camponotides, and much less attention has been paid to their habits and intelligence. Forel, until recently, adopted four groups: Myrmicini, Attini, Pseudomyrmini and Cryptocerini; but he is now disposed to increase this number to eight.[67] They are distinguished by differences in the clypeus, and in the form of the head; but it must be noted that the characters by which the groups are defined are not in all cases fully applicable to the males. The Cryptocerini are in external structure the most highly modified of Hymenoptera, if not of all the tribes of Insecta. [Illustration: Fig. 67—_Pheidologeton laboriosus_, large and small workers. East India.] i. The Myrmicini proper are defined by Forel as having the antennae inserted near the middle, a little behind the front, of the head, which has carinae on the inner sides, but none on the outer sides, of the insertions of the antennae; the clypeus extends between the antennae. [Illustration: Fig. 68—_Formicoxenus nitidulus_, male. (After Adlerz.)]