Feb. 4, 1918 Interrelations of Fruit-Fly Parasites in Hawaii 287 



attacks the young of O. humUis, usually at a point a few segments back 

 of the head, the point of contact being clearly seen in Plate 12. 

 Its mandibles open wide and snap into the body of the attacked larva 

 spasmodically and with remarkable quickness. Often the entire opera- 

 tion of broadly opening and closing the mandibles may be almost imper- 

 ceptible to the eye. It may move its entire body quickly, the caudal 

 tip may be curled beneath the body and extended again suddenly, and 

 the mandibles may be repeatedly opened and closed until successfully 

 grasping the Opius larva. Besides possessing unusual powers for inflict- 

 ing injury to other parasitic larvae about it, it may avoid counterattack 

 through ability to move quickly and through the protection afforded 

 the entire ventral surface of the body by a thick mass of serosal, cellular 

 material that accompanies the larva when it emerges from the egg, and 

 which remains with it during its entire life in the first instar. This mass 

 of cells may be seen clinging to Diachasma larvae of the first instar in 

 Plate 10. 



The newly hatched larva of Opius humilis possesses mandibles which 

 are also long and pointed, as shown in Plate 12, A, B. These may 

 be used to good advantage when the larva is successful in bringing them 

 in contact with individuals of its own or of other species of parasites. 

 The larva, however, is sluggish, moves much less quickly than that of 

 Diachasma tryoni, is protected ventrally by a much thinner, less adhesive 

 mass of serosal cells, is much less capable of quick and powerful move- 

 ment of the mandibles, and usually holds the body in a somewhat hori- 

 zontal and exposed position. These deficiencies seem to explain its 

 inability to avoid destruction by larvs of D. tryoni or to offer successful 

 counterattack when the two are lodged within the same host larva. 



During the examination of nearly 3,000 fruit-fly larvae or pupae, para- 

 sitized in each case by the two species of opiines, the dead or dying and 

 often struggling Opius larvae were frequently dissected from the tissues 

 of the host while still tightly clasped in the mandibles of the Diachasma 

 larvae. Plate 10, A, is reproduced from a photomicrograph of a larva 

 of O. humilis actually within the grasp of an active, living larva of 

 D. tryoni. In this particular case the operation of removing the two 

 larvae from the host, placing them upon a microscope slide in water, 

 and covering them with a cover glass failed to separate them, and the 

 Diachasma larva ultimately died with its mandibles deeply embedded 

 in the body of the dead Opius larva in the exact position as shown. 

 Plates 12 and 13, A, B, C, show larvae of O. humilis in various stages of 

 laceration and distortion just as they were removed from fruit-fly larvae 

 or pupae in which were also one or more larvae of D. tryoni. 



Extensive laboratory experiments have exactly confirmed the results 

 of the first series of field observations. The aggressive, cannibalistic 

 period of activity of the larvae of these parasites is during existence in 



