VIII. Hymenoptera.
Cynipidae. I have been unable to find any accurate accounts of the duration of life in the imagos of saw-flies or ichneumons; but on the other hand I owe to the kindness of Dr. Adler, an excellent observer of the Cynipidae, the precise accounts of that family which are in my possession. I asked Dr. Adler the general question as to whether there was any variation in the duration of life among the Cynipidae corresponding to the conditions under which the deposition of eggs took place; whether those species which lay many eggs, or of which the oviposition is laborious and protracted, lived longer than those species which lay relatively few eggs, or easily and quickly find the suitable places in which to deposit them.
Dr. Adler fully confirmed my suppositions and supported them by the following statements:—
‘The summer generation of Neuroterus (Spathegaster) has the shortest life of all Cynipidae. Whether captured or reared from the galls I have only kept them alive on an average for three to four days. In this generation the work of oviposition requires the shortest time and the least expenditure of energy, for the eggs are simply laid on the surface of a leaf. The number of eggs in the ovary is also smaller than that of other species, averaging about 200. This form of Cynips can easily lay 100 eggs a day.
‘The summer generation of Dryophanta (Spathegaster Taschenbergi, verrucosus, etc.) lives somewhat longer; I have kept them in confinement for six to eight days. The oviposition requires a considerable expenditure of time and strength, for the ovipositor has to pierce the rather tough mid-rib or vein of a leaf. The number of eggs in the ovary averages 300 to 400.
‘The summer generation of Andricus, which belongs to the extensive genus Aphilotrix, have also a long life. I have kept the smaller Andricus (such as A. nudus, A. cirratus, A. noduli) alive for a week, and the larger (A. inflator, A. curvator, A. ramuli) for two weeks. The smaller species pierce the young buds when quite soft, but the larger ones bore through the fully grown buds protected by tough scales. The ovary of the former contains 400 to 500 eggs, that of the latter over 600.
‘The agamic winter generations live much longer. The species of Neuroterus have the shortest life; they live for two weeks at the outside; on the other hand, species of Aphilotrix live quite four weeks, and Dryophanta and Biorhiza even longer. I have kept Dryophanta scutellaris alive for three months. The number of eggs in these agamic Cynipidae is much larger: Dryophanta and Aphilotrix contain 1200 and Neuroterus about 1000.’
It is evidently, therefore, a general rule that the duration of life is directly proportional to the number of eggs and to the time and energy expended in oviposition. It must of course be understood that, here as in all other instances, these are not the only factors which determine the duration of life, but many other factors, at present unknown, may be in combination with them and assist in producing the result. For example, it is very probable that the time of year at which the imagos appear exerts some indirect influence. The long-lived Biorhiza emerges from the gall in the middle of winter, and at once begins to deposit eggs in the oak buds. Although the insect is not sensitive to low temperature, for I have myself seen oviposition proceeding when the thermometer stood at 5° R., yet very severe frost would certainly lead to interruption and would cause the insect to shelter itself among dead leaves on the ground. Such interruptions may be of long duration and frequently repeated, so that the remarkably long life of this species may perhaps be looked upon as an adaptation to its winter life.
Ants. Lasius flavus lays its eggs in the autumn, and the young larvae pass the winter in the nest. The males and females leave the cocoons in June, and pair during July and August. The males fly out of the nest with the females, but they do not return to it; ‘they die shortly after pairing.’ It is also believed that the females do not return to the nest, but found new colonies; this point is however one of the most uncertain in the natural history of ants. On the other hand it is quite certain that the female may live for years within the nest, continuing to lay fertilized eggs. Old females are sometimes found in the colony, with their jaws worn down to the hypodermis.
Breeding experiments confirm these statements. P. Huber[[17]] and Christ have already put the life of the female at three to four years, and Sir John Lubbock, who has been lately occupied with the natural history of ants, was able to keep a female worker of Formica sanguinea alive for five years; and he has been kind enough to write and inform me that two females of Formica fusca, which he captured in a wood together with ten workers, in December 1874, are still alive (July 1881), so that these insects live as imagos for six and a half years or more[[18]].