Fig. 152—Proterhinus lecontei. Hawaiian Islands. A, Male; B, female; C, front foot, more magnified.
Strepsiptera (or Rhipiptera, Stylopidae).—Male small or minute; prothorax extremely small; mesothorax moderate, the elytra reduced to small, free slips; metathorax and wings very large; nervuration of the latter radiating, without cross nervules. Female a mere sac, with one extremity smaller and forming a sort of neck or head.
Fig. 153.—Sexes of Strepsiptera. A, Male of Stylops dalii (after Curtis); B, female of Xenos rossii (after von Siebold).
These curious Insects are parasitic in the interior of other Insects, of the Orders Hymenoptera and Hemiptera. Their structure and their life-histories entitle them to be ranked as the most abnormal of all Insects, and entomologists are not agreed as to whether they are aberrant Coleoptera or a distinct Order. The newly-hatched larva is a minute triungulin (Fig. 154), somewhat like that of Meloe; it fixes itself to the skin of the larva of a Hymenopterous Insect, penetrates into the interior, and there undergoes its metamorphoses, the male emerging to enjoy a brief period of an abnormally active, indeed agitated, existence, while the female never moves. It is important to note that these Strepsiptera do not, like most other internal parasites, produce the death of their hosts; these complete their metamorphosis, and the development of the parasite goes on simultaneously with that of the host, so that the imago of the Strepsipteron is found only in the imago of the host.[[158]] After the young Stylops has entered its host it feeds for a week or so on the fat-body (apparently by a process of suction), then moults and assumes the condition of a footless maggot, in which state it remains till growth is completed. At the latter part of this period the history diverges according to sex; the female undergoes only a slight metamorphic development of certain parts, accompanied apparently by actual degradation of other parts; while the male goes on to pupation, as is normal in Insects. (We may remark that the great features of the development of the sexes are parallel with those of Coccidae in Hemiptera.) When the Hymenopterous larva changes to a pupa, the larva of the Strepsipteron pushes one extremity of its body between two of the abdominal rings of its host, so that this extremity becomes external, and in this position it completes its metamorphosis, the male emerging very soon after the host has become an active winged Insect, while the female undergoes no further change of position, but becomes a sac, in the interior of which young develop in enormous numbers, finally emerging from the mother-sac in the form of the little triungulins we have already mentioned. This is all that can be given at present as a general account; many points of the natural history are still obscure, others have been merely guessed; while some appear to differ greatly in the different forms. A few brief remarks as to these points must suffice.
Bees carrying, or that have carried, Strepsiptera, are said to be stylopised (it being a species of the genus Stylops that chiefly infests bees); the term is also used with a wider application, all Insects that carry a Strepsipterous parasite being termed stylopised, though it may be a Strepsipteron of a genus very different from Stylops that attacks them. The development of one or more Strepsiptera in an Insect usually causes some deformity in the abdomen of its host, and effects considerable changes in the condition of its internal organs, and also in some of the external characters. Great difference of opinion prevails as to what these changes are; it is clear, however, that they vary much according to the species, and also according to the extent of the stylopisation. Usually only one Stylops is developed in a bee; but two, three, and even four have been observed:[[159]] and in the case of the wasp, Polistes, Hubbard has observed that a single individual may bear eight or ten individuals of its Strepsipteron (Xenos, n. sp.?).
Fig. 154—Young larva of Stylops on a bee's-hair. Greatly magnified. (After Newport.)
There is no exact information as to how the young triungulins find their way to the bee-larvae they live in. Here again the discrepancy of opinion that prevails is probably due to great difference really existing as to the method. When a Stylops carried by an Insect (a Hymenopteron, be it noted, for we have no information whatever as to Hemiptera) produces young, they cover the body of the host as if it were powdered, being excessively minute and their numbers very great; many hundreds, if not thousands, of young being produced by a single Stylops. The species of the wasp genus Polistes are specially subject to the attacks of Stylops; they are social Insects, and a stylopised specimen being sickly does not as a rule leave the nest; in this case the Stylops larva may therefore have but little difficulty in finding its way to a Hymenopterous larva, for even though it may have to live for months before it has the chance of attaching itself to a nest-building female, yet it is clearly in the right neighbourhood. The bee genus Andrena has, however, quite different habits; normally a single female makes her nest underground; but in the case of a stylopised female it is certain that no nest is built, and no larvae produced by a stylopised example, so that the young triungulins must leave the body of the bee in order to come near their prey. They can be active, and have great powers of leaping, so that it is perhaps in this way possible for them to attach themselves to a healthy female bee.