642 MR. G. S. COTTERELL ON THE LIFE-HISTORY 



The first segment is divided into two oral lobes with radiating 

 food-channels on their ventral surfaces as in the preceding instar. 



The chitinised mouth -parts are similar in general structure to 

 those of the second instar, but are proportionately stronger. The 

 later-al pharyngeal sclerites are longer and more deeply cleft 

 (text-fig. 9, a). The intermediary sclerites are much thickened 

 and shortened. The buccal sclerites (text-fig. 9, d) have no 

 similarity witli those of the second instar. They have lost the 

 spoon-shaped structure, and are now a pair of stout ventrally 

 curved hooks. They are pointed anteriorly and thickened where 

 they articulate with the intermediary sclerites. They project 

 externally over the oral opening. 



The anterior spiracles (text-fig. 9) are situated laterally at the 

 junction of the second and third segments. They consist of a 

 short chitinised trunk, jDrojecting forwards and externally from 

 the junction of the segments. The trunk divides into two lobes, 

 each lobe bearing usually eight papillae, making sixteen in all. 

 Each papilla is pierced by a small lumen for the ingress of air. 

 The number of papillae varies from sixteen to eighteen in difierent 

 larvse. 



The posterior spiracles (text-fig. 10) are situated side. by side in 

 the middle of the anal plate. Each consists of a chitinised ring, 

 situated on a short projection, enclosing three slit-like apertures. 

 The apertures are bordered by inwardly projecting filaments 

 serving as a sieve for the incoming air. These spiracles are the 

 most necessary to the larva, being those it keeps above the surface- 

 film when living in very liquid dung. The respective anterior 

 and posterior spiracles are connected by two lateral longitudinal 

 tracheal trunks. The lateral trunks are connected, soon after 

 their origins from the posteiior spiracles, by a transverse trunk. 

 In addition the longitudinal trunks give o& small branches in 

 each segment. 



The anus is situated on the triangular plate on the ventral 

 surface of the last segment. This plate is devoid of spines, and 

 appears to be glandular in structure. 



The third instar period takes from 6 to 9 days, pupation 

 taking place on the ninth to twelth day of larval life. 



The larva is active only during the first two days of this instar, 

 after which it seeks the drier parts of the dung or the soil, 

 preferably the latter. During the inactive period it changes 

 from a more or less transparent appearance to an opaque one, 

 due to the great development of the fat-body. 



Great difiiculty was experienced in determining whether the 

 whole of the chitinised mouth-parts were thrown oft' at ecdysis. 

 Owing to the thin epidermis and the impossibility of finding cast 

 skins in the dung, a set of old mouth-parts w?,s never found. 

 Theoretically the complete set should be thrown oft", and recently 

 I had the good fortune to mount a preparation within an hour 

 or so of the first moult showing a complete second set outside 

 the old set. Up to this time I had proof that the buccal and 



