30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



ing lateral abdominal gills ( D ) the caudal appendages are large vesicu- 

 late lobes. Ris (1912) describes them as ovoid vesicles with thin outer 

 walls, and an internal alveolar structure penetrated by blood canals 

 and tracheae, but he draws no conclusion as to their function. Tillyard 

 says the saccoid gills are formed by inflation of the triquetral form. 

 It is said that even the appendages that are evidently gills in function 

 may be broken off without any apparent effect on the activity of the 

 mutilated individual. It would seem, therefore, that the caudal ap- 

 pendages of the zygopterous larva may have been first develo|)ed per- 

 haps as locomotor organs and were later modified in most species for 

 respiratory purposes. 



Zygopterous larvae are said by Tillyard (1917) to have a supple- 

 mentary rectal respiration, which is particularly vigorous in a newly 

 hatched larva, and which in larvae that have lost their caudal gills 

 "can be watched by a careful use of carmine particles." By "respira- 

 tion" evidently is meant an inhalation and exhalation of water through 

 the anus. The walls of the rectum, Tillyard says, are thrown into 

 three longitudinal folds of high columnar cells, but the folds are not 

 tracheated and can serve only as blood gills. It may be supposed, then, 

 that intestinal respiration was first tried out as a substitute for external 

 branchial respiration, and was later perfected in the anisopterous 

 group by the development of tracheated folds within a prerectal sac, 

 while in the others the caudal appendages were modified for respira- 

 tion, though in a few forms the primitive lateral gills were retained. 

 Since use of the respiratory sac for locomotion is a specialty of 

 anisopterous larvae, and depends on abdominal contraction, it prob- 

 ably followed as a secondary function of the sac. 



At the transformation of the larva to the adult, both the skeleton 

 and the musculature of the abdomen undergo a complete reorganiza- 

 tion. The simple arched segmental tergum of the larva (fig. 9F) 

 becomes much flattened in the adult (fig. 10 A) with a ridge along 

 the dorsal midline, and the lateral parts sharply folded onto the ven- 

 tral surface in two sections, first horizontally, and then dorsally to 

 form the lateral walls of a deep median channel containing the ster- 

 num (S). The broad, three-part larval sternum is replaced in the 

 imago by a narrow median plate (B, ^), widened anteriorly where it 

 bears on each side two lateral apodemal processes, and tapered pos- 

 teriorly to an apical point. The laterosternites of the larva have en- 

 tirely disappeared as such, and the spiracles of the adult (Sp) are 

 contained in membranous conjunctivae between the sterna and the 

 inflected tergal margins. 



The strong dorsal, ventral, and lateral musculature of the larval 



