INTRODUCTION . 7 



tion, N. Wagner points out that the vermiform larvoB 

 of certain small gnats have reproductive power to carry 

 on the series. In Diptera the larvae are footless grubs. 



With reference to the mandibulate and haustellate 

 types of mouth, Sir J. Lubbock says " In Campodea a 

 type of animal occurs closely resembling certain larvas 

 which occur both in the mandibulate and suctorial 

 series of insects." On this account it is urged that 

 many difficulties would vanish by an adoption of Cam- 

 podea (Thysanura) as an ancestral type of Insecta.* 



Conformity to conditions and occasional reversions 

 to ancestral tjrpes are now generally accepted biological 

 facts, and by their consideration we may expect a good 

 intellectual harvest. In the present volume some 

 remarkable examples of degradation are shown — cases 

 in which the visual organs are entirely aborted,! others 

 where they are in process of development during meta- 

 morphoses of the insect, and only show their complete- 

 ness in the imago. Similarly, the mouth-organs are 

 found to be aborted in the sexes of other species. 

 Once more we may find indications of a possible rever- 

 sion to an earlier tjrpe in some simpler forms of an 

 early age. 



We may ask. Is the slightly endowed subterranean 

 Aphis more nearly allied to some presumed ancestral 

 form, but little differentiated; or should we consider it 

 to be a degradation from some higher genus like 

 Siphonophora or Drepanosiphon ? If we argue that 

 old forms are foreshadowed in embryology, the 

 Aphididas should come from a far-seeing stock; for eyes 

 appear in very early stages within the egg. 



Thus, in Trama shall we trace, according to the 

 suggestions of Professors Lankester and Balfour, the 

 acquisition of eyes from a local deposit of pigment, 

 which through the stimulus of light and heat raises the 



* ' Monograph of the Collembola,' Ray Society, 1873, p. 52. 



t Examples of Aphides without eyes may be found in Trama, PI. CII, 

 fig. 6 ; in Schizoneura, PI. OVI, figs. 4 — 9 ; Pemphigus, PL CX, figs. 

 3—8 ; PI. CXI, fig. 8 ; PI. CXII, fig. 7. In other underground larva) 

 the eyes, if present, are very small. 



