Fig. 79.—Forked spine of Limnobia, enlarged; the dark lines show air-tubes.
The pupa is quite as remarkable as the larva, though the breathing apparatus does not assume the form of spines, but consists of two ‘trumpets,’ one on each side of the head, as is the case with the pupae of gnats, using that term in a wide sense. The colour is a greenish-brown, dotted with black. The abdomen is capable of a good deal of motion from side to side; and by this means the pupa can raise itself to the surface of the water to take in a supply of air.
De Geer remarked that when the pupa rose for this purpose it lay with its body horizontal, having the lips of the trumpets a little above the surface. It appeared not to like being placed on its back, because in that position the trumpets cannot be raised above the surface. When he tried the experiment of putting the pupa back downwards, it wriggled over by bending the abdomen.
Fig. 80.—Pupa case of Limnobia.
(From a photograph taken at the Yorkshire College, Leeds.)
On the abdomen there are several chitinous hooks, which serve in some sort as substitutes for limbs. By their means the pupa can moor itself to the stems of aquatic plants; and this is necessary, since its specific gravity being less than that of the water it would be always at the surface, if it had no such means of anchorage. And of course, at the top of the water, it would be exposed to the danger of being snapped up by birds.
De Geer’s specimen passed six days in the pupal state, and then emerged as a perfect Crane-fly. My specimens did not emerge till after ten and eleven days of pupahood, which seems strange, as they were plentifully supplied with food in their larval stage.
The fly is a little more than half an inch long, and may be distinguished from the common Daddy-longlegs by the character of the wings, which are folded somewhat like those of a wasp, but with this difference: that the wings of this fly are folded outward, while those of the wasp are folded inwards. When the insect wishes to fly, it opens the fold so that the whole wing presents a plane surface, but the fold reappears directly the Crane-fly ceases its flight (Frontispiece).