Huber contrived that his bees should be unable to build in their usual instinctive manner, beginning from above and working downwards; on this they began building from below, and again horizontally. The outermost cells that spring from the top of the hive or abut against its sides are not hexagonal, but pentagonal, so as to gain in strength, being attached with one base instead of two sides. In autumn bees lengthen their existing honey cells if these are insufficient, but in the ensuing spring they again shorten them in order to get greater roadway between the combs. When the full combs have become too heavy, they strengthen the walls of the uppermost or bearing cells by thickening them with wax and propolis. If larvæ of working bees are introduced into the cells set apart for drones, the working bees will cover these cells with the flat lids usual for this kind of larvæ, and not with the round ones that are proper for drones. In autumn, as a general rule, bees kill their drones, but they refrain from doing this when they have lost their queen, and keep them to fertilise the young queen, who will be developed from larvæ that would otherwise have become working bees. Huber observed that they defend the entrance of their hive against the inroads of the sphinx moth by means of skilful constructions made of wax and propolis. They only introduce propolis when they want it for the execution of repairs, or for some other special purpose. Spiders and caterpillars also display marvellous dexterity in the repair of their webs if they have been damaged, and this requires powers perfectly distinct from those requisite for the construction of a new one. The above examples might be multiplied indefinitely, but they are sufficient to establish the fact that instincts are not capacities rolled, as it were, off a reel mechanically, according to an invariable system, but that they adapt themselves most closely to the circumstances of each case, and are capable of such great modification and variation that at times they almost appear to cease to be instinctive.