408 THE INVERTEBRATA 



was involved. Embryological evidence in support of this conclusion 

 is of a doubtful nature, and the most acceptable view to take is that 

 the superlinguae are processes attached to the hypopharynx and 

 perhaps homologous with the paragnaths of Crustacea. 



Order COLLEMBOLA (Springtails) 



Small wingless insects with biting mouth parts deeply withdrawn 

 into the head ; compound eyes absent ; 6-segmented abdomen which 

 often carries three pairs of highly modified appendages serving the 

 purposes of adhesion and jumping ; a tracheal system is commonly 

 absent and there are no Malpighian tubules; metamorphosis absent. 



Four-jointed antennae, ocelli and postantennal sensory organs are 

 characteristic features of the head. 



There are no tarsi on the legs, claws being borne by the tibiae. The 



Fig. 304. A, Axelsonia (Collembola). B, Hamula of Tomoceros showing 

 c. basal piece, and r. its rami. From Imms, after Carpenter and Wilson. 

 p. ocular pigmented area; v. ventral tube; h. hamula; m., d. and nic. caudal 

 furcula. 



I St abdominal segment carries a ventral tube which is moistened by 

 a glandular secretion from behind the labium poured down a ventral 

 groove running along the middle of the thorax. This ventral tube, re- 

 garded as adhesive, is formed by the fusion of the embryonic append- 

 ages of this segment. On the ventral side of the 3rd segment, the nearly 

 complete fusion of a pair of appendages has resulted in the formation 

 of the hamula, which engages Xht furcula prior to leaping. The latter 

 is a forked structure representing a pair of limbs of the 4th segment 

 (Fig. 304). By contraction of the extensor muscles of the furcula 

 the latter is pulled down out of contact with the hamula and the 

 animal is propelled forwards into the air. 



The absence of tracheae is a secondary feature due to the small size 

 of the animals rendering surface respiration sufficient for their mode 

 of life. 



