51�Թ�

Haustellum, haws-tel′um, n. the sucking organ or proboscis of an insect or a crustacean:—pl.

Antlia: the spiral tongue or haustellum of Lepidoptera.

Haustellum: a sucker: applied to that portion of the mouth of a sucking insect through which liquid food is drawn into the gullet.

We will suppose that a change takes place in a particular orchid, that the nectary recedes to a greater distance from the point to which the insect can penetrate, and so an advantage is given to those insects in which the haustellum is of a length above the average.

The insect derives an advantage from its increased haustellum, but what advantage does the plant derive from its retiring nectary?