212 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



thus providing "a priori" for the inevitable failure of the efforts of a 

 large number of individuals. Furthermore it can be admitted that this 

 larva, having achieved its aim, could have no more reason for keeping 

 its features. Now we cannot deny, in a larva of the caterpillar type, 

 having the thoracic legs transformed into membranous pseudolegs and 

 the abdominal pseudopodes fitted for attaching an hectophagous larva 

 to the victim's body, that the tracheal spiracles emerging on promi- 

 nences prevent the haemolymph, running from the voluminous body 

 of the victim, from penetrating into the tracheae, or that the mouth 

 parts fitted for tearing and sucking are correlated with an haemato- 

 phagous diet. It is obvious, however, that from another point of view, 

 we are in the presence of a rather odd plasticity, which seemingly 

 has no appreciable meaning in the light of what we know at present. 

 Rhipidius quadriceps Ab. lives on the preimaginal stages of several 

 species of Blattoidea belonging to the genus Ectobius Steph. The 

 female lays eggs on the bark of plants. The larva of the first phase 

 does not wait for the host, but goes and looks for it without delay. 

 Having reached it, the larva attacks the intersegmental corium, into 

 which it wedges its head and a portion of its thorax. After 2 to 3 

 weeks a new very odd larval instar, which in a special way is injected 

 into the victim's body cavity, issues from this larva which has practi- 

 cally remained ectozoan. There it passes the summer, fall, and winter 

 in diapause (growing little or not at all). In April it is transformed 

 (through a kind of transition instar which feeds actively) into the 

 last larval phase, which remains within the host up to the moment of 

 undergoing metamorphosis ; then it emerges and pupates outside. The 

 Rhipidius larva of the first phase (triungulinum) is like the larva of 

 the Rhipiphorini. The larva of the last phase is very different from 

 that of this subfamily; its mouth parts are formed by only one pair 

 of appendages, considered to be maxillary palpi ; the mouth is physio- 

 logically closed and there are three pairs of well-developed legs (it 

 must walk to where it will pupate). Of the two intermediate stages, 

 the interesting instar is the one injected into the victim's body cavity 

 by the triungulinum ; this is an apodous, indistinctly jointed form with 

 a membranous integument, without antennae, mouth parts (although 

 it is not astomous), and stigmata. In the cycle it takes the place of the 

 replete form of the Rhipiphorini's triungulin. It is clear that the dif- 

 ferent cycles and behaviors of the various preimaginal instars, except 

 the triungulins that have similar tasks, exhibit strongly differentiated 

 structures. 



