A relation of this bird, the Colaptes mexicanus, does not yield to him in economy and skill. He places his barn in the interior of a plant which is very abundant in the zone he inhabits. Insectivorous during a part of the year, he is forced to renounce this diet during the dry season. In the regions of Mexico where this bird is found the dry period is so absolute that he would die of hunger for want of insects or fruits if he had not taken the precaution of laying up stores during spring. His store consists of acorns. He has not time to fix them one by one, like the Melanerpes, and only thinks at first of rapidly collecting a large quantity. But it is in deciding the question as to where they are to be laid up that the Colaptes shows his remarkable intelligence. In the forests where he lives are to be found aloes, yuccas, and agaves. When the agaves have flowered, the flower-bearing stem, two or three metres in length, shrivels, but remains standing for some time. Its peripheral portion is hardened by the heat, while the sap in the interior almost entirely disappears. A hollow cylinder with a well-sheltered cavity is thus formed, and the Colaptes proposes to utilise it as a storehouse. His acorns will there be well protected against external influences and against the birds whose beaks are too weak to pierce the agave. It is then a question of filling the tube. The animal first pierces the wall towards the base of the stalk; through this hole he introduces acorns until he has filled the lower part of the cavity. This done, he makes a new hole rather above the first, and fills the interval between the two, continuing this process until he has arrived at the top of the stalk and filled the whole interior. ([Figs. 10 and 11.]) The bird seems at first to take unnecessary trouble by boring so many holes. He would reach his end as well, it would seem, by making a single hole at the top to fill his storehouse, and another at the bottom to empty it. But we must not thus accuse him of lack of judgment. The interior of the tube is just large enough for the passage of an acorn; but at certain points the sap is not entirely absorbed, and there might easily be an impediment which would leave a large part of the cavity empty. Hence the necessity for a number of openings. When the sun has scorched up plants, and provisions are rare, he turns to his barns of abundance. Now and every time that he has need he can utilise the method that has been employed by his cousin the Melanerpes. In order to feed on each acorn without too much trouble, or allowing it to slip from his beak, the bird places it in a vice. He hollows a hole in the trunk of a tree, introduces the fruit there forcibly, and eats it at his ease.[52]
The provisions collected by these two birds reveal a remarkable fact. They possess indeed two distinct diets; they do not preserve for the period of famine the overplus of the foods which they consume in the period of abundance. They chase insects and feed on them as long as they can find them, while they gather up in their storehouses an entirely different food.
Physiological reserves. — All the animals of which I have just spoken place their provisions for the future in barns in the same manner as Man. Those who have not this foresight are either able to nourish themselves in all seasons by the chase, or else, after having feasted one half of the year, they fast during the other half. In the latter case they consume during the fasting period a portion of their own substance, and use up materials placed in reserve in their organism, in the form of fat for example. This arrangement, which allows them to prolong life, though growing thin, until the next season of prosperity, is not under the control of the will. It is a complication of physiological phenomena resulting from the functioning of different parts of the organism.
Stages between physiological reserves and provisions. — Between physiological reserves and industrial stores we may place as an intermediate stage the interesting case of the Honey Ants.[53]
These insects (Myrmecocystus) live in Texas, and form colonies in which certain individuals play a very special part They exaggerate to an extreme point the power of preserving provisions in their crops. These materials are not assimilated; they do not form part of the animal’s body, and although placed inside it cannot be compared to physiological reserves. It is especially curious that they are not to be utilised only by the animal itself, but also by the other members of the colony who are not able to form such stores. Among the Myrmecocystus there are workers of two sorts; the first kind resemble other ants with some differences of detail, and build and hollow the earth nest which shelters the community. The second kind is quite different; the abdomen in these workers is enormously distended so as to constitute a voluminous sphere, which may become four or five times larger than the thorax and head together. ([Fig. 12.]) On this distended receptacle appear several darker plates; these are the remains of the chitinous parts of the primitive wings. In the fine season these ants go out in a band and collect a sweet liquor which forms pearly drops on certain galls of oak leaves. These drops, elaborated into honey, gradually fill the crop, distending it and pushing back neighbouring organs until it receives its globular form. When they have arrived at this obese condition, the heavy honey ants no longer leave the nest. They remain without movement, hanging by their legs to the roof or lying against the walls of a room. The workers who have remained slender come and go, attending to their usual occupations, and pass near the others without paying attention to them or going out of the way to lend assistance to their impotent sisters when one of them has rolled over on the ground and can no longer arise unaided. ([Fig. 13.]) They only cease to be indifferent when impelled by the selfish sentiment of hunger, and then it is to ask and not to give assistance. The fat ants in fact could not themselves consume all the honey that they have elaborated; the others in times of famine approach them, caress them with their antennæ, and obtain by solicitation a drop of honey which the large ones disgorge from the crop. Here, then, is a colony in which the division of labour has reached a remarkable degree of polymorphism. Some of the members accomplish the work of engineers and masons, while the others fabricate for the community a store of honey. Instead of depositing these provisions in cells like bees, they preserve them in their own digestive tube. This custom has re-acted to such an extent on the form of their bodies that at first sight they seem to belong to a different species.
