Popular Science Monthly/Volume 50/November 1896/Evolution of Insect Instinct
|EVOLUTION OF INSECT INSTINCT.|
I WAS a witness in 1887 of a combat between a halictus bee and its sphœcode parasite, a "cuckoo bee," which took place in the open air, outside of the nest. The nests of the Halictus malachurus (Kirby), which are found excavated in the compact soil of garden walks, are narrower at the entrance than below, and here the sentinel bee closes access with its head.
The sphœcode, Sphœcodus hispanicus (Wesmäel), twice as large as its victims, had to enlarge this entrance to effect its passage. I saw it cut up the sentinel, whose quarters came out with the digging. Very near, a halictus was assisting a dying sister whose pollen-loaded feet were still moving. She had without doubt been killed by the sphœcode. Another harvester still survived, and attacked the parasite, biting its legs and wings. The bandit, obliged to stop its task frequently, established itself near the nest and tried to seize the enemy with its sharp mandibles. The halictus at last threw itself upon him, and the two were locked in combat. In an instant the halictus was no more.
The sphœcode labored for nearly four hours to open a passage, and would perhaps have succeeded if I had not judged it prudent to capture it. It had worked till dark without having advanced more than an eighth of an inch.
Besides the deductions which other authors have drawn from the observation of insects under similar conditions to these, I found a no less important feature toward the study of instinct in the apparent development at the same time with sociability of a courage which impels the individual to devotion of itself to the common cause. The persistent struggle which my halictus maintained is, I believe, unexampled in the annals of other Hymenoptera than ants, wasps, honeybees, and bumblebees. It was not a rush of a moment upon the thief, or a struggle in a narrow corridor where escape was impossible after the fight had begun; but it was a foot-to-foot battle that lasted nearly a quarter of an hour, in the open field, where the halictus could run away at any moment. The assault was made vigorously, of determined purpose, the contestants fighting in close embrace, and ended in the death of one of them.
We remark that this devotion to the public weal is like society in its beginning. It is less developed here than among the wasps and the honeybees. Near the one who died so bravely on the field of honor, I saw a new halictus alight in front of the enemy at the first attack to go take care of the dying. Honeybees do not hesitate when their city is to be defended, and bumblebees, Hoffer says, often precipitate themselves upon the man who is destroying their nest; but their civilization, if I may use the word, is much more advanced than that of the halictus. Courage and abnegation are therefore not only the appanage of mankind or of rich societies of honeybees and ants; they belong to every association—to all those, whether beasts or people, who bind their hearts together in the struggle for existence.
The tenacity of the sphœcodes on the field of battle is not less surprising; it is, so far as I know, the only example of a parasite issuing from the peaceful progeny of the apiaries that gives battle for the acquisition of spoil.
Have we here a species of parasite in course of formation? I do not know. There is a great distance between the sphœcode and the halictus. The variation of the genus or species sphœcodes is very great, it is true; but that of the bumblebees is of the same order, and there are fossil bumble bees. The hazardous life of the parasite should teach us reserve concerning the cause of its variations.
I believe we may observe a nascent parasitism in another family of Hymenoptera. I mean among those insects which honestly gain life for their young most of the time, but which also do not disdain to rob a neighbor, to play the parasite, and that not fortuitously, but almost every time an occasion presents itself. I have found such insects in the spider-killing family of the Pompilidœ.
The pompiledes are those little black wasps, with a somewhat party-colored abdomen, which may be seen lingering on sunny talus or walls, with their antennae and wings in febrile vibration. Those that I have observed in France and Algeria chase spiders. They pursue them, keeping in touch with the ground like a dog following game. The manner of attack varies with the species of the hunters. Nearly all those that I have seen light directly upon the enemy, which rolls over, and stab it. The spider is generally put in a safe place on top of a tuft or a stone, while the pompilus digs a hole in which it deposits the anæsthetized head, after gluing its egg upon the abdomen. The pompilides are not all diggers—some choose or prepare the most singular places for their progeny; but the general rule is as I have described it.
My observations have been made chiefly upon the Pompilus viaticus (Latreille) and on the Pompilus rufipes (Vanderlinden). If we throw a spider that has been stung by a fellow-wasp at one of these, it will nearly always be taken without hesitation, and will often be stung again. The depositing and the laying of the egg are done as if by habit; and I may add that the hunter is not dainty as to the freshness of his game. I have seen spiders of eight days' standing accepted, and have repeated the experiment so often that I can not suppose that the fact is accidental. It is not a case of one opening a cell to deposit an egg because its own has been stolen, or of digging into the partitions at the end of its labors; but what I relate happens almost regularly whenever occasion offers. It may be said that the insect is obliged to deposit its egg. Perhaps, but the necessity for ovipositing is singularly elastic with my pompili, and is associated with the faculty they have of stealing the game of their neighbor.
A Pompilus viaticus has just drawn its spider into the cell. It has deposited its egg and stopped up its hole. I offer it a new spider, killed; it is not the time for ovipositing, but the victim is accepted and placed carefully by the side of the nest, the closing of which is arrested. A new cell is dug out, the booty is drawn into it, and receives an egg in its turn.
I have often repeated this experiment with Pompilus viaticus and pectinipes. I broke open the half-closed nest, and unfastened the egg, and I have several times seen the spider taken up, carried a little farther on, and the ovipositing begun again.
So far I have told of experiments; now I come to pure observation. Let us go at the beginning of September into a warm gravelly quarry. We see many hymenoptera there, but the pompilides dominate. They have chosen the most agreeable quarter, the most sunny one in the city. Those which I observed were the Pompilus rufipes. They are a colony of crafty fellows, constantly in motion, ferreting everywhere, sometimes on the quest for a neighbor's spider, going into the holes which they find to their taste to drive the proprietors from them. When they have succeeded in stealing, they bury their spoil, if some other thief does not interfere, and deposit an egg upon it. These thefts are often the occasion of lively combats. I chanced to see two of the largest of the band disputing over a spider. Hunters and victim rolled like a ball along the gravel for four or five yards. The contestants, which had not let go, tugged at their prey like dogs wrangling over a bone. After a few minutes the beaten one—generally the less corpulent—gave up the struggle. The species, however, is not parasitic. The spider is in the beginning the legitimate prey of one of the two, and I have, besides, seen them hunting and ovipositing honestly in the same quarry.
Not only in the capture of the prey, but in the choice of the nest, too, a very great adaptation of instinct to conditions exists among the pompilides. They turn everything to profit.
Taschenberg says that the Pogonius nest In the sand. I have found Pogonius bifasciatus nesting in a hedge at Châtellerault in abandoned snail shells. Some shells contained as many as three cocoons. This year, at Algiers, I found bulimus containing cocoons which have not hatched at this writing, but which strongly resemble the cocoons of my pogonius. If Taschenberg has not made a mistake, the insect is a digger that does not always dig. I have long observed a little pompilus at Châtellerault which I have not been able to identify. I have seen it nesting almost everywhere—in snail shells, in the rotten mortar of old walls, and in worm-eaten wood, digging when it had no other way. One day it even had the audacity, while we were at lunch, to bring its spider to my sister's hair.
We are therefore, it seems to me, contemplating an eminently variable instinct, which, joined to the tendencies to parasitism of which I have just spoken, suggests that a parasitical branch may be even now detaching itself from the pompilus type.
The pompilides, or some among them, have possibly been showing these tendencies for many centuries. The walks of the garden near Algiers are crowded in October with small spiders which pass the day hidden in holes closed by a stone or a clod. I have observed that a little Salius knew very well how to open this retreat, go in, and kill the inmate. Prof. Pérez, in his contributions to the apian fauna of France, has studied the parasites of bees in a masterly manner, but he has almost omitted the study of instinct in the formation of parasitism. I have no more than suggested the question, but I believe we might easily give an acceptable answer to it with the help of the pompilides. If we succeed in this, we shall perhaps have answered the challenge sent out in his Souvenirs by the entomologist of Sérignan: "Let them show me a species in the course of transformation."—Translated for the Popular Science Monthly from the Revue Scientifique.
- Die Hymenoptera Deutschland, etc.