Microscopic as the wheel animalcules are they yet have been watched and examined by their admirers to as great a point of intimacy as that reached by the devotees of insects or of birds. A remarkable fact about them is that in about 130 different species (out of the 500 known) it has been found that the males are diminutive creatures, about one-tenth the size of the females, and devoid of digestive canal; in fact, little more than minute swimming sacs full of spermatozoa. In one group, that of the crawling Rotifers, to which the common wheel animalcule, figured in the last chapter, belongs, no male at all has ever been discovered. They are all females. They are precisely those wheel animalcules which are known to microscopists for their power of surviving (like the little water-bears or tardigrades and some other minute animalcules) the desiccation, or "drying-up" of the water in which they were living, swimming, and crawling (see Chapters XV. and XVI.). And it is quite probable that this power of resistance to the adverse conditions of changing seasons has, in the crawling Rotifers, taken the place of the production of eggs fertilized by a male. For, as in the case of the crustacean water-fleas (and of the terrestrial plant-lice, or aphides and gall-flies), it is found that the female Rotifers or wheel animalcules, which hatch from fertilized eggs, are themselves "parthenogenetic," and lay eggs which develop without fertilization by males–that is to say, are "impaternate." In the case of the water-fleas these are called "summer eggs," and after one or more generations of such fatherless females a proportion of males are produced which fertilize the females hatched at the same period. The eggs so fertilized acquire a thick shell and are called "winter eggs." They remain dormant for some months and resist the injurious influences of winter cold, or, it may be, of drying up and conversion of the pond-mud into dust, but hatch out when warmer and wetter conditions return.