to extend along the limbs for some distance and to settle in the lymphatics of the groin and axillæ. If it be true, as has been reported, that the ancient Mexicans tortured or even killed their enemies by binding them to ant-nests, P. barbatus was certainly the species employed in this atrocious practice. It is commonly supposed that the poison responsible for the pain inflicted by these and other ants is formic acid, but chemical analysis of P. molefaciens by Melander and Brues (1906) failed to reveal any traces of this substance. Hence the poison of this insect must be some unknown substance, possibly a nucleoalbumin. This confirms the opinion of other authors, who, like Fürth (1903), deny that the general physiological effects of the sting, even in the European ants, are due to formic acid.

CHAPTER XVII.

THE RELATIONS OF ANTS TO VASCULAR PLANTS.

"Plantas itaque norunt formica."-Michael Gehlerus, 1610.

"Die Ergründung der interessanten Gemeinschaft zwischen Pflanzen und Ameisen wurde bisher als ein rein botanisches Problem aufgefasst. Aber gerade hierin liegt-wenn mir ein Urteil in dieser Frage zugestanden werden solltedie Ursache der geringen Erfolge, um nicht zu sagen der Misserfolge. Man war stets allzusehr geneigt, die Anpassungsfähigkeit der Pflanzen an plötzlich eintretende, sie berührende Verhältnisse, als erheblich hinzustellen, umgekehrt aber glaubte man diejenige von lebenden Wesen mit so achtungsgebietender Begabung, wie solche den Ameisen eigen ist, übergehen zu müssen; der Instinkt dieser findigen Tiere täuschte eben über den nur höchst schwerfällig arbeitenden Anpassungsmechanismus der Pflanze hinweg." - Rettig, Ameisenpflanzen und Pflanzenameisen," 1904.

The hypothesis of intimate mutualistic relations between ants and the higher plants is one of those fascinating constructions in which certain gifted and imaginative botanists have rivalled the inventors of the mimicry hypothesis in the zoological field. Both of these constructions have been treated as facts of the utmost value in supporting a still more general hypothesis-that of natural selection, and both, after having been carried to extremes by their respective adherents, are now facing the reaction that is overtaking Neodarwinism. Authors like Fritz Müller, Schimper, Huth, Delpino, Beccari and Heim have marshalled a formidable array of observations in favor of the view that many plants develop elaborate structures to be used as lodgings by certain pugnacious ants or even furnish these insects with exquisite food substances, and in return for these services are protected by their tenants from the leaf-cutting ants or from other leaf-destroying animals. These observations are now being subjected to critical revision by authors like Rettig and H. von Ihering, whose attitude towards the whole subject is avowedly skeptical and reactionary. It behooves us therefore to examine both sides of the argument and, if possible, to adopt a position which will favor and not forestall further investigations.

We may divide our subject into two parts and consider, first, the plant adaptations that are said to indicate symbiosis, and second, the ants that are associated with plants. The supposed adaptations may be considered under two heads: the dwelling places and the food provided for the ants. The former consist either of preformed cavities or of structures from which the pith or loose tissue can be easily removed

and thus converted into habitable tenements.

In both cases the cavity

is entered through a small orifice which either preëxists or is made by the ants. This orifice then constitutes the nest opening, or entrance. These simple requirements are fulfilled by a great many plant structures which therefore make admirable domiciles for small ants that live permanently in small colonies or for incipient colonies of larger ants that later form populous communities. The rigid vegetable tissues are an excellent protection against enemies, and the cavities are moist, dark and free from moulds, so that they make perfect nurseries for the larvæ and pupæ. Cavities of this description are especially utilized by ants. in the tropics, probably because there these insects are more abundant and the struggle for existence is keener. The following paragraphs will show how numerous and variable are the plant organs that may be tenanted by ants:

1. Cavities in Stems.-Almost any hollow or pithy stem, with resistant walls sufficiently thin

B

D

to be pierced by ants, may be entered and occupied by these insects. Some plants, however, are especially well-suited to these purposes, for example, those of the Old World genera Randia, Myristica, Clerodendron, Kibara and Bambusa, with preformed cavities, and Endospermum and Juglans with pithy stems; among the New World genera: Cecropia, Triplaris, Tachigalea, Humboldtia, Tachia, Ficus, Cordia, Duroia, Coussapoa, Pterocladon, Pterocarpon, Bombax, Cladium, etc., with preformed cavities, and Coccoloba uvifera, Sapium, Schwartzia, Platymyschium and Sambucus with pithy stems. The entrances to the cavities are actually foreshadowed as pits in the internodes in Cecropia and Clerodendron.

A

FIG. 166. Stems of "myrmecophilous plants. (Escherich, after Schimper.) A, Ficus inæqualis; B, Triplaris americanus; C, same, older stem; D, Humboldtia laurifolia; x, entrance to cavity of stem.

2. Tubers, Bulbs, Pseudobulbs, Rootstocks, etc.-Many examples could be cited under this head. The most celebrated are the Malayan Rubiaceous epiphytes of the genera Myrmecodia and Hydnophytum,

which have large pseudobulbs full of preformed cavities, nearly always tenanted by ants. Certain ferns of the genus Lecanopteris from the same region, and certain orchids of the genera Grammatophyllum and

FIG. 167. The "Palo Santo" (Triplaris boliviana) in fruit, from a specimen in the herbarium of the New York Botanical Garden. (Original.)

Schomburgckia are also cited as providing accommodations for ants. in their pseudobulbs.

3. Ascidia or Burse of Leaves and Petioles.-The straight or convoluted leaf-petioles of certain pitcher plants (Nepenthes bicalcarata)

are often hollowed out and inhabited by ants (Heim), and the curious ascidia of Dischidia rafflesiana are similarly utilized. In North America various species of ants inhabit the old, dry pitchers of Sarracenia. In

FIG. 168. Stem and leaf of Endospermum formicarum, the former inhabited by colonies of Camponotus quadriceps. (Dahl.) Two nest openings are seen in upper part of stem. Where the petiole joins the leaf there are two nectaries.

South America several genera of plants (Tococa, Majeta, Microphyscia, Calophyscia, Myrmedone, Hirtella) have bladder-like dilatations of