NO. 8 ANATOMICAL LIFE OF THE MOSQUITO — SNODGRASS 27 



adult with little alteration. The rudiments of the future wing muscles, 

 however, are present in the thorax during the last larval instar. Hulst 

 (1906), on the other hand, reports that in the larva of Culex there 

 takes place an extensive histolysis and histogenesis of the body mus- 

 culature, beginning when the larva is two-thirds grown. Some imaginal 

 muscles thus appear first in the larva, particularly those of the wings 

 and legs, prior to the advent of the pupal stage. Destruction of the 

 larval muscles of the abdomen, however, Hulst says, is not complete 

 even in a late stage of the pupa. In the Simuliidae, according to 

 Hinton (1959), "the indirect flight muscles and the tergal depressor 

 of the trochanter develop quite independently of the larval muscles 

 in all post-embryonic stages." 



Histological changes in the alimentary canal beginning in the larva 

 have been described by Samtleben (1929), by Berger (1938) for 

 Culex, and by Richins (1945) for Aedes. The replacement of func- 

 tional cells from regenerative cells in the ventriculus during larval life 

 is generally in other insects not a metamorphic process but the usual 

 procedure of replacing worn-out digestive cells by new cells. At the 

 fourth ecdysis to the pupa, however, Berger (1938) says, rapid 

 changes take place. The alimentary canal of the pupa, well illustrated 

 by Hurst (1890), differs from that of the larva, but is still not that 

 of the adult. The short pupal stomach is said by Richins to be formed 

 from only the posterior part of the larval stomach. According to 

 Samtleben no specific pupal epithelium is formed for the pupal 

 stomach. 



Considering the precocious development of so many of the imaginal 

 organs, the fourth instar of the mosquito larva presents the anomalous 

 condition of being part larval and part pupal. In other words, the 

 pupal development begins within the larva long before its completion 

 at the pupal ecdysis. It ends with the formation of the pupal head, 

 mouth parts, and tail fins. 



In most young insects the endocrinologists find that the larval struc- 

 ture is maintained by the inhibitory action of the corpus allatum hor- 

 mone on the adult development until the end of the larval life. The 

 early origin of pupal organs in the mosquito larva and the continu- 

 ance of their development through the larval period shows, however, 

 that the juvenile, or status quo, hormone does not necessarily function 

 as a complete inhibitor of adult development. In the mosquito it ap- 

 pears to be selective in its action, allowing the growth of pupal parts 

 that do not interfere with the normal activities of the larva, while it 

 maintains to the end of the larval period such parts as the head, feed- 



