Should all the rooms be available, a rare occurrence, there still remains a method of distinguishing between an ancient nest and a recent one. The cocoon, as I have said, adheres pretty closely to the walls; and the mother does not always take away this remnant, either because she is unable to do so, or because she considers the removal unnecessary. Thus the base of the new cocoon is set in the bottom of the old cocoon. This double wrapper points very clearly to two generations, two separate years. I have even found as many as three cocoons fitting one into another at their bases. Consequently, the nests of the Mason-bee of the Pebbles are able to do duty for three years, if not more. Eventually they become utter ruins, abandoned to the Spiders and to various smaller Bees or Wasps, who take up their quarters in the crumbling rooms. As we see, an old nest is hardly ever capable of containing the Mason-bee's entire laying, which calls for some fifteen apartments. The number of rooms at her disposal is most unequal, but always very small. It is saying much when there are enough to receive about half the laying. Four or five cells, sometimes two or even one: that is what the Mason usually finds in a nest that is not her own work. This large reduction is explained when we remember the numerous parasites that live upon the unfortunate Bee. Now, how are the sexes distributed in those layings which are necessarily broken up between one old nest and another? They are distributed in such a way as utterly to upset the idea of an invariable succession first of females and then of males, the idea which occurs to us on examining the new nests. If this rule were a constant one, we should be bound to find in the old domes at one time only females, at another only males, according as the laying was at its first or at its second stage. The simultaneous presence of the two sexes would then correspond with the transition period between one stage and the next and should be very unusual. On the contrary, it is very common; and, however few cells there may be, we always find both females and males in the old nests, on the sole condition that the compartments have the regulation holding-capacity, a large capacity for the females, a lesser for the males, as we have seen. The old male cells can be recognized by their position on the outer edges and by their capacity, measuring on an average the same as a column of sand 31 millimetres high in a glass tube 5 millimetres wide. (1.21 x.195 inches.--Translator's Note.) These cells contain males of the second or third generation and none but males. In the old female cells, those in the middle, whose capacity is measured by a similar column of sand 45 millimetres high (1.75 inches.--Translator's Note.), are females and none but females.