CHAPTER XXII.

TRUE GUESTS, ECTO- AND ENTOPARASITES.

"Es gibt wohl wenige Gebiete der Zoologie, wo Morphologie und Biologie sich so nahe berühren, so innig durchdringen und sich gegenseitig so ergänzen wie hier: die Biologie erschliesst erst das volle Verständniss der betreffenden morphologischen Charaktere, und andererseits lassen die morphologischen Charaktere uns oft bereits die Biologie jener Wesen vorauserkennen und geben uns die wichtigsten Winke für die Erforschung derselben."-Wasmann, "Die Myrmekophilen und Termitophilen," 1896.

The persecuted and tolerated guests described in the last chapter are merely the plebeian precursors of the aristocracy among the myrmecophiles, the 300 or 400 true guests, which are no longer content to be treated with animosity or indifference, but have acquired more intimate and even friendly relations with the ants. These true guests are not, therefore, to be found skulking in the unfrequented galleries of the nest, or suspiciously dodging about among the ants, but live in their very midst with an air of calm assurance, if not of proprietorship. As a rule, they have abandoned such indefinite or panmyrmecophilous attachments as those of the synceketes and have settled down to associations with particular host species or genera. The ants, however, still remain the passively exploited partners of the alliance; they become, in fact, only the more easily mulcted and despoiled as the symbiotic intimacy increases, till, in some cases, they seem to be suffering from a social obsession or disease like the alcoholism of human communities. It is but a step from these true guests, or symphiles, to the parasites in the restricted sense. Some have regarded the symphiles, like the synechthrans and the synceketes, as parasites on the ant colony, in contradistinction to the ecto- and entoparasites, which exploit the individual ant or ant larva, but this, as we shall see, is a somewhat artificial distinction.

The Symphiles.-These are very largely beetles, and though they belong to many different families, they show a remarkable adaptive convergence, for in order to solicit food from the ants and ingratiate themselves by means of peculiar exudations, they have developed the following peculiarities in coloration, in the structure of glands, mouthparts and antennæ:

1. Symphilic Coloration.-Wasmann has called attention to the peculiar red color and oily surface characteristic of many true guests.

This is best seen in the Lomechusini, Clavigeride and Paussidæ, but is also apparent in many other species (certain Cremastocheilus, Silphidæ, Thorictidæ, etc.) which are, perhaps, degenerate symphiles. The exact signification of this symphilic coloration is not known.

2. Trichomes.-Several of the earlier students of myrmecophily (Erichson, Lacordaire, W. P. J. Müller) observed that the true guests, as a rule, bear tufts of red or golden yellow hairs (trichomes, or

FIG. 236. Various species of Paussida. (Wasmann.) A, Pleuropterus brevicornis; B, Paussus hova; C, Pentaplatarthrus natalensis; D, Paussus dama; E, Lebioderus goryi; F, Paussus spiniceps.

trichodes) which are assiduously licked by the ants, and much has been made of these structures by Wasmann, who regards them as the most characteristic organs of symphiles. He has shown that they are borne by the chitinous integument at points or depressions where clusters of unicellular glands open, and that they have the important function of rapidly diffusing some aromatic secretion. Glands of a similar type, from which the trichomes may have developed, are present in many

nonmyrmecophilous insects, e. g., the epinotal glands of the ants themselves, the dorsal glands of the Blattidæ, the scent glands of Lepidoptera, etc. Wasmann states that the secretion is not liquid, but " volatile or etherial, perhaps a fatty ether." The ants are so inordinately fond of it that he believes that it must affect them very much as a good cigar affects a smoker. Perhaps it would be nearer the truth to say that its fascination is more like that exercised by catnip or oil of bergamot on the various members of the cat family. Wasmann summarizes the distribution of the trichomes, which may be developed on almost any part of the body, as follows: "On the sides and base of the portions of the abdomen not covered by the wingcases (Lomechusa group of Staphylinids, many Clavigerids); on the tip of the abdomen (Lomechusa group), or pygidium (many Paussus); at the tips of the wing-cases (many Clavigerids, Chatopisthes among termitophilous Scarabæids); on the sides of the wing-cases (many Paussus); at the posterior corners or edges of the prothorax (Pleuropterus and many Paussus among Paussids, Lomechon among Silphids, Corythoderus and Chatopisthes among termitophilous Scarabæids, Tylois among Histerids, many Thorictus among Thoric

tids); on the anterior corners of the prothorax (Napochus termitophilus); on the much elevated sides of the prothorax (Teratosoma among Histerids, Gnostus among Gnostids); in a median transverse groove on the prothorax (many Paussus); on the neck, between the head and prothorax (the myrmecophilous Napochus among the Scydmænidæ, Tetramopria among Proctotrupids); on a perforated horn on the vertex (several Paussus); on the front (Pogonoxenus among Tene

brionids); on the antennal club (many Paussus); and even on the coxæ and tips of the femora (Lomechusa)."

3. Mouthparts.-The symphiles are not only licked, but also fed by the ants. This has led to a peculiar shortening and broadening of the tongue, its fusion with the paraglossæ and a reduction in the number and size of the joints of the labial palpi, in many symphiles. These modifications make the tongue a more spoon-like organ, adapted to receiving the liquid regurgitated by the ants.

4. Antenna.-In the symphiles these organs have, in many cases, undergone peculiar modifications, depending upon whether they are used primarily as organs of communication, of transportation, or of protection. In communication they may have a two-fold function, since they may be employed either in making supplicatory movements in order to induce the ants to regurgitate, or the movements may be of

such a character as to deceive the ants into mistaking the beetles for other members of the colony. Space forbids an adequate account of the wonderful variety of structures in the antennæ of the symphiles. Suffice it to say that the antennæ adapted for stroking the ants approach those of the Formicidæ in structure and movement; others have the joints fused and dilated (Paussus) and are used as handles, by means of which the ants can carry or drag their guests about the nest. Such antennæ are obviously protective, but there are also protective antennæ of another type which are short, compact and spindle-shaped, so that they slip through the jaws of the ants. Such antennæ are best developed in certain synceketes and synechthrans and may be said to have a function the very opposite of communication.

The symphiles, as already stated, are nearly all Coleoptera. They belong to the families Paussidæ, Clavigeridæ, Pselaphidæ, Thorictidæ, Cossyphodidæ, Silphidæ, Nitidulidæ, Ectrephidæ, Gnostidæ, Scydmænidæ, Staphylinidæ, Brenthidæ and Tenebrionidæ. As it will be impossible here to describe or even to enumerate the various species, I shall confine myself to some of the more striking and interesting types.

Of all myrmecophilous insects the Paussidæ (Fig. 236) are the most extraordinary. They are an aberrant offshoot of the Carabidæ, and, with the exception of two species of Homopterus, that have been taken in equatorial South America, are all paleotropical. Although nearly 300 species are known, the behavior of very few has been carefully observed, and the accounts are so different as to indicate a wide range of myrmecophilous habits within the family. The group, on account of the variety and bizarre structure of its species, is a favorite one with coleopterists. It has been studied by Boyes (1843), Westwood (184345), Dohrn (1851), Gueinzius (1858-'59), Peringuey (1883, 1886), Raffray (1885-'87, 1892), Wasmann (1888, 1890g, 18949, 1896h, 18979, 1904c, etc.) and Escherich (1898, 1907). The salient characters of the family are found in the antennæ which vary in the number of their joints from II to 2. In the latter case the distal joint is formed by a fusion of several. Wasmann divides the family into four groups of genera according to the number of joints: first, Protopaussus with II joints; second, Arthropterus, Homopterus, Orthopterus, Cerapterus and Pleuropterus with 10 joints; third, Ceratoderus, Merismoderus and Pentaplatarthrus with 6, and Paussoides with 5 joints; and fourth, Lebioderus, Platyrhopalus, Paussomorphus, Paussus and Hylotorus with two joints. Several of these groups, of which Paussoides is one, were already represented in the Prussian amber (Lower Oligocene). These beetles are not only anomalous in the number of joints in the antennæ, but also in the form of these organs, which in some species are broad and elliptical, in others shaped like ribbons, antlers, scimitars, drumsticks, boats, etc. Westwood described Paussus sphærocerus as having phosphorescent antennæ "like two lanthorns spreading a dim phosphoric light," and Wasmann believes that this observation is correct, as the two spherical antennal clubs in a cabinet specimen examined by him had a peculiar yellow color like the phosphorescing spots on the thorax of Pyrophorus. The remarkable development of trichomes in some of the species and their absence in many others indicate that these beetles are not all symphiles, though they are probably all myrmecophiles. In some forms, like Paussus cucullatus, the tufts of golden hairs may be present on the pygidium, in the median thoracic groove, in the double frontal pits and in the clefts of the antennal clubs.

In