On a New Type of Respiratory Interrelation between an Insect (Chalcid) Parasite and its Host (Coccidae)

Abstract

1. The life history ofEncyrtus (Comys) infelix(Embleton), a hymenopterous parasite belonging to the chalcid family Encyrtidae, is described. It parasitisesSaissetia hemisphaerica, a member of the subfamily Lecaniinae (Homoptera Coccidae). Its respiratory relationships with the host are of a quite extraordinary character.2. The egg is provided with a hollow stalk which is left protruding from the posterior dorsal body wall of the host. The first three larval instars are metapneustic, the spiracles being placed at the tip of a pair of long caudal processes which are inserted in the hollow stalk. It has been proved that these spiracles are open and in actual communication with the atmospheric air, which enters through the pedicel.3. The fourth and fifth larval instars are amphipneustic and the manner of respiration is entirely changed. The caudal processes degenerate and finally break away. The fourth instar larva turns round in the scale insect and becomes invested with a closely fitting transparent membraneous sheath produced by the phagocytes and fine tracheal branches of the host. This process appears to be merely a special case of the type of phagocytic activity which normally gives rise to the basement membranes and “connective tissue” membranes; such membranes are particularly conspicuous and well developed inSaissetia. The sheath becomes attached in an extraordinary manner to the main lateral tracheal trunks of the host in four (or six) places in the neighbourhood of the larval spiracles. This process is described in detail. It has been proved by an injection technique that an actual connection is established between the lumen of the host trachea and the cavity of the sheath. Bubbles of gas appear inside the sheath close to the points of attachment and near the spiracles of the larva which are thus put into functional communication with the tracheal system of the host.4. While the production of the sheath might under certain circumstances be of value to the host the development of tracheal attachments appears to be of value to the parasite alone. The conclusion that the whole structure is an adaptation for the respiration of the parasite seems inescapable. A theory is tentatively put forward to account for the stimulation of the host tracheal epithelium by a sudden physiological change in the tension of the respiratory gases in the region of the parasite spiracles.5. References to similar instances among the Chalcids are briefly reviewed. It is suggested that the “puparia” which have been described in the case of one or two other Chalcidoidea may be found to arise in the same way.