398 The University Science Bulletin. 



LARVA. 



Length, 6-7 mm. when fully grown. Color white. 



Head very small, not over .25 mm. in diameter. Prothoracic proleg double, 

 short, with many short spines; caudal prolegs short, each with a ring of about 

 24 short spines. Anal gills not apparent. 



Head yellowish brown, with a narrow black hind border; on the ventral 

 surface of the head, near the hind border, there is on each side a light oval 

 spot, suggestive of an ocellus. Mouth parts very small and difficult to dis- 

 sect out; t-hey are much reduced in size; the mandibles are composed of a 

 single sharp tooth with two short spines on the inner margin near the base 

 of the tooth; labrum small, the front margin excavated in the middle so as 

 to leave a blunth' rounded chitinized tooth on each side; labial palpi very 

 short : antennae minute, easily overlooked. 



PUPA. 



Length, 5-5.3 mm. Color brown; a narrow, blackish margin around the 

 wings, and a narrow longitudinal dark line each side of the abdomen. The 

 -^ntire pupa is smooth and devoid of any vestiture. Breathing trumpets not 

 well developed, but represented by a spiracularlike chitinized area each side 

 of the mesothorax. These breathing trumpets are best seen in the late 

 larval stage when the pupa is being formed (fig. 7). Segment one to three 

 of the abdomen smooth; segments four and five, above, each with a double 

 transverse band of fine spines on the posterior margin, the spines of the hind- 

 most bands directed forward and at least twice as long as the spines on the 

 band immediately in front ; segments six, seven and eight) each with a single 

 transverse band of short spines. Genital sacs smooth, no hairs or spines 

 present. On the lateral margin of the abdominal segments the vestigial 

 spiracles are visible. 



THE BIOLOGICAL RELATIONSHIP 



Between Trissocladms equitans and Rithrogena sp. 



With plentiful material on hand it has been possible to establish 

 the relationship which exists between the immature stages of these 

 insects. The larva of Trissocladius equitans is parastic upon the 

 Rithrogena May-fly nymph. Although it has not been possible to 

 learn in what manner the larva establishes itself as a parasite on the 

 nymph, indications are that Trissocladius equitans spends its entire 

 larval life as a parasite upon the May-fly nymph. Different-sized 

 larvae, representing the instars, except possibly the first, were found 

 under the wing pads of the nymphs. 



The parasite attaches itself to the posterior margin of the meso- 

 thorax underneath the wing pads, where it imbeds its head in the 

 softer tissue of the nymph and where it draws its nourishment from 

 the host. The larva spins a sheet of silk, which completely invests 

 its body, and by means of which it attaches itself firmly to the 

 body of the nymph. At first, stretched to its full length, the larva 



