Page:Phylogeny of cynipid genera and biological characteristics.pdf/29

 Again, agamic reproduction has evident advantages but it is inconceivable, from what we know of reproduction among animals, that such a method of propogation, especially where agamy has become the only method, can result in anything less than a loss of vitality and the ultimate extinction of the species. This belief may be due to our inadequate knowledge of biological phenomena, but it is significant that the agamic species of cynipids (e. g., Rhodites, Disholcaspis, and the genera with an alternation of generations) more often fail to reach maturity than do species of the bisexual groups (Aulacidea, Diastrophus, etc.). It is a common experience to obtain only a score of adults from hundreds of the oak- or rose-galls, while many hundreds of adults may be secured easily from a score of galls of Aulacidea, for instance. The failure of those certain species to reach maturity may be due in part to their higher degree of parasitism, but it is not due entirely to that.

Still another thing which indicates decreased vitality in the oak gall-wasps is the short duration of the adult life of most species. Having observed some thousands of adults of scores of species on trees until the insects died natural deaths, I found that about two days was the average length of life for those species, while many species will die almost instantly after oviposition, even if that is only a few hours after the emergence from the gall.

The adults of most species of cynipids are very weak creatures, fatally injured by the slightest touch or by a sudden change of temperature or of humidity. One of the greatest obstacles in my experimental breeding has been the enormous amount of destruction caused if a rainy or even foggy day occurred at the time of emergence of a lot of adults. Although I kept the insects under cover, at such a time they would not oviposit or even make any attempt to climb over the trees; when the weather would become favorable in a day or two most of the wasps had lost all vitality and soon died without laying eggs.

Still another indication of the reduced vitality of most of these cynipids is the number of adults which reach maturity but then do not succeed in making (chewing) their way through the walls of the gall. It is hard to determine what percentage of adults thus fail, at the last moment, to achieve the goal of their lives, but it is apparently large, for almost any collection of galls, cut open, will show many such individuals in galls which were evidently mature before they were gathered. For instance, from one lot of galls of Andricus pomiformis not a single individual was bred, although at least 150 entirely mature adults were found withi-n the galls. Osten Sacken bred only one adult from the type