172 INSECT TRANSFORMATION 



A young spring-tail newly-hatched, differs from its parent 

 in no important respect. Among the modifications common 

 within the order, a reduction of the spring is strongly charac- 

 teristic in certain genera, and this may be carried so far that 

 the organ becomes a mere vestige or vanishes entirely. This 

 retrogressive change in the spring can be traced in some 

 spring-tails in the course of the life-history. For example, 

 in a Triacanthella, the adult shows no junction between 

 dens and mucro, but the newly-hatched young may possess 

 a spring in which these parts are clearly recognizable (Fig. 99) 

 recalling the condition found in allied genera such as Achorutes 

 which have the various elements of the spring developed 

 normally, though remaining relatively small. 



FIG. 99. ABDOMINAL APPENDAGES OF SPRING-TAIL ( 

 alba), CAMPBELL ISLAND. 



a, " catch " ; 6, spring of adult viewed obliquely (paired 

 denies without mucrones) ; c, d, spring (side view) of very 

 young ; (c) and half-grown (d) specimens ; in the former is a 

 distinct mucro (m) which disappears in later stages, x 280. 

 After Carpenter, " Sub-antarctic Islands of New Zealand ". 

 XVII. 



Summing up the facts briefly reviewed in this and previous 

 chapters, we see that secondary winglessness is a condition 

 that has appeared again and again in various orders of insects, 

 in some cases associated with parasitism or some abnormal 

 mode of life, in others rather with an abundant food-supply 

 available without the necessity for migration as with the 

 wingless generations of virgin aphids, and possibly in the 

 experimental series of Drosophila. The wingless condition 

 of the adult may be correlated with a normal type of life- 

 history, either exopterygote, as in the case of the aphids and 

 the lice, or endopterygote, as in the case of the fleas. On 

 the other hand such wingless parasites as Melophagus, exhibit 

 a most unusual type of life-history, though one which can be 

 seen also among their winged relations, while in Termitoxenia 



