Chapman then goes on to say that finally, within six hours of the time of spinning up of the wasp grub, the Rhipiphorus larva at the end of Stage 1., which is “usually in motion, and for its situation might be called tolerably active, is seen to lay hold of the interior of the skin with its anterior legs, and keeps biting and scratching with its strong and sharp jaws until it is able to thrust through its head, when, in less than a quarter of an hour, it completely emerges by a vermiform movement; and at the same time it casts a skin, together with the black head, legs, plates, etc.”
The larva, now in its second stage, passes forward and seizes hold of the upper or lateral aspect of the prothoracic segment of the wasp grub. On emerging it becomes shorter and thicker, “and very soon loses length by that curving forward of its head which is so marked in the full-grown larva, and which does not exist before its emergence.” The larva is now found “lying like a collar immediately under the head of the wasp grub, and is attached to it by the head, though not very firmly.” At this stage the feeding of the young Rhipiphorus is rather sucking than eating.
Fig. 646.—First larva (a) of Bruchus fabæ, greatly enlarged; b, thoracic processes; c, head, from front; d, from side; e, antenna; f, thoracic leg; g, rear view of tarsus; h, same, front view.—After Riley.
When about 6 mm. in length it moults a second time, and the full-grown larva closely though superficially resembles a Crabro or Pemphredon larva, the small head being bent over forwards. By the time it is ready to pupate it has wholly eaten the wasp larva, and the temperature of the cell being high, a larva 5 mm. long grows large enough in two days to fill the top of the cell of its host, and the larva is ready to pupate about a week after hatching, so that its development is very rapid. The beetles themselves do not live in the cells. Chapman thinks they hibernate, and that the eggs are laid in the spring or summer.
We thus have in this insect three larval stages, the triungulin, and two later stages, the great differences between the first and the last two being apparently due to their parasitic mode of life, the larva spending its second stage within its host, involving an existence in a cell with a high temperature, an uninterrupted supply of rich, stimulating food, and a comparatively sedentary mode of life compared with that of the triungulin at the beginning of its existence. It is quite obvious that the hypermetamorphosis is primarily due to a great change in its surroundings, i.e. the parasitic mode of life of the beetle, habits of very rare occurrence in the Coleoptera, numerous in species as they are.
Fig. 647.—First larval stage of Bruchus pisi: a, egg in pea-pod; b, cross-section of opening of mine; c, young larva and opening on inside of pod by which it has entered, enlarged; d, d, d, eggs, natural size; e, 1st larval stage; f, a leg of same; g, prothoracic spinous processes.—After Riley.
In this connection attention may be drawn to a supernumerary larval stage observed by Riley in the pea- and bean-weevils (Figs. 646 and 647). The larva on hatching has long slender legs, though differing from those of an ordinary coleopterous larva in having but three joints (j, g, h). This stage is very short, and the legs temporary, as, after entering the bean or pea, it casts its skin, losing its legs, and assuming the vermiform shape of the second larval stage. In this case the change from a pedate to an apodous larva is plainly enough due to the change from an external feeder, like a chrysomelid larva, to a larva leading a boring, internal, almost quiescent life.
Certain ichneumons also appear to have two distinct larval stages, as Ratzeburg inferred that in Anomalon there are four larval stages (Fig. 648).