Insect identification > Strepsitera

Strepsitera


These tiny insects are seldom seen except by entomologists, and their parasitic habits aid in their concealment. For a long time opinions were divided as to where they belonged, some regarding them as a family of aberrant Coleoptera, while others considered them as forming an order. Recent studies seem to confirm the latter view and the group is now generally rated as a separate order, though its closest relations are probably with the beetles.

The Strepsiptera, from the meaning of this name, may be called the twisted-wing parasites, though the words stylops and stylopid are frequently used in referring to them. The males, on reaching the adult condition, become free and can fly. The females, on the other hand, remain partly within the bodies of their host insects and are worm-like or grub-like in appearance. The males are very small, soft-bodied animals, ranging from about one to perhaps four twenty-fifths of an inch in length.

The eyes are more or less stalked and the antennae have one or more segments elongated on one side. The mouth-parts are greatly modified but appear to be of the chewing type, though the adult does not feed. On the mesothorax is a pair of tiny clubs, sometimes rather flattened, which represent the front pair of wings. The metathorax forms nearly half the entire length of the body. It bears a pair of well-developed wings which are broad and fold lengthwise when at rest. The abdomen is composed of 10 segments.

The females are soft and resemble a rather long sack bearing traces of segmentation, and at one end is a constriction, beyond which is a sort of knob, believed to be a combination of the head and thorax; a cephalothorax in fact. This portion of the body is pushed out between two of the body segments of the host during the latter part of the metamorphosis, thus becoming external and the body of the host is distorted in this way.

The members of this order may be characterized as follows:
Tiny insects which from the first larval instar to the adult are internal
parasites in other insects. The male adult has stalked eyes, mouth-parts of the chewing type, but little or not at all developed; antennae with one or more segments prolonged laterally; pro- and mesothorax small, the latter with a pair of small clubs corresponding to the forewings of most insects; meta­thorax long, forming at least half the length of the body and bearing a pair of broad wings which fold longitudinally. The female adult is worm-like, without feet, and located within the body of its host except for a cephalothorax which protrudes between two abdominal plates of the latter. It is enclosed by its pupa skin. Metamorphosis complete.


These insects, often called "stylops," are parasitic only in some Orthoptera, Homoptera, Hemiptera and Hymenoptera, as far as known, and at the present time only Gryllotalpa in the Orthoptera and Chrysocoris in the Hemiptera are known as hosts in those groups. Most of the parasitism is of leafhoppers, wasps and the solitary bees, and these are so disabled by the removal of their body fluids by the parasites that "stylopized"individuals are unable t0 reproduce and are greatly lacking in vitality. Their bodies are often distorted also and other segments are produced.

The eggs of the stylops appear to hatch within the body of the mother and the young escape by passing from the body out into the space between this and the pupa case of the parent in which it remains, and then through an opening in this at the cephalothorax, thus reaching the open air. They are now on the body of the parental host and this insect may carry them to its nest, where, if the host is a colonial form, the stylops may find young to attack there. It is generally probable, though, that they leave the parental host at some place (possibly a blossom) where other insects of the host species will be liable to visit.

Transferring onto such individuals as chance may permit, the stylopids finally arrive where larvae of the proper species are available and at once attack them. Thus far they have been active little six-legged larvae, but after burrowing into the body of their host larvae they change greatly, becoming worm-like and legless. The males finally enter a pupa stage, after which the adults escape, but the females remain throughout the rest of their life in the bodies of their hosts.

Where stylopids are abundant and attack injurious species of insects, such as are most, at least, of the Homoptera, the stylopized individuals, being unable to produce, become of lessened importance and their parasites must be considered as beneficial. Most of the Hymenoptera they attack, however, are beneficial and parasitism in such cases can hardly be considered helpful to man. The group is not sufficiently abundant, though, to be an important factor under ordinary conditions, as only about a hundred species are known, but these are widely distributed over the globe.