188 ME. E. B. POULTON ON THE EXTEENAL 



one part of the same subject, viz. the external reproductive organs, although the prin- 

 cipal part of his investigation has been concerned with the development of the internal 

 organs. Under these circumstances we both agreed that it would be advisable to publish 

 at the same time and through the same channel. The two papers will thus supplement 

 each other — Mr. Jackson's supplying the details of internal anatomy at various stages of 

 development, mine dealing with the external organs in a number of different species. 

 For this reason the first paper of my series is chiefly concerned with the external repro- 

 ductive organs. I may add that I had intended to work out the internal anatomy by 

 means of sections and dissections, but, in the press of other work, such an investigation 

 might have caused the indefinite postponement of publication ; I am therefore especially 

 pleased that Mr. Jackson should have been led to undertake this inquiry. 



The Names of the various Appendages Sfc. of the Pupa. — It has been the custom 

 hitherto to speak and think of the various parts of tlie pupa as if they were mere cases 

 for the corresponding part of the imago. Thus the terms oj^hthalmothecce, pterothecce, 

 ceratotheccp, poclothecce, dfcc. have been applied to the parts within wliich the imaginal 

 eyes, wings, antennae, legs, &c. are respectively contained. The investigations which 

 will be described in this series of papers have convinced me that these terms and ideas 

 are entirely erroneous. Such appendages or organs represent parts of the pupa, and I 

 shall speak of them as pupal eyes, wings, antenna?, legs, &c. Although modified in 

 shape, so that the imaginal organs can be contained within them, their form and struc- 

 ture are not identical with the latter, but are far more ancestral ; they are remnants of a 

 time when the last stage of metamorphosis in the ancestors of Lepidoptera was something 

 very difterent from a bu-tterfiy or moth. The old terminology obscured the fact that the 

 pupa has a morphological meaning of its own, and that traces of an extremely remote 

 past can be deciphered by the study of its structure. 



It is well known that the pupa can be dissected out of the slun of a mature larva 

 manv hours before the occurrence of normal pupation. Under these circumstances the 

 pupal appendages are not soldered down by a thick coat of varnish, which hardens on 

 exposure to the air, but stand out freely as evident legs, wings, &c. These appearances 

 are nevertheless unaccountably described by many authors as the appendages of the 

 perfect insect. Thus Swammerdam poitits out the method by which the pupa can be 

 freed from the larval skin in Fieris brassicce; he then says : — " This done, it is clearly 

 and distinctly seen that within this skin of the caterpillar a perfect and real butterfly 

 was hidden" (' Book of Nature,' ii. 26). This erroneous view is corrected by Sir John 

 Lubbock *. 



Professor Weismann's great discovery that the contents of the pupa of Diptera break 

 down (histolysis) into nutrient fluids and lowly differentiated units, from which the 

 imago is subsequently built up by a process akin to embryological development, has an 

 important bearing upon the subject. If we examine a section of a pupal antenna or leg 

 (in Lepidoptera) we shall find that there is no trace of the corresponding imaginal organ 

 until shortly before the emergence of the imago. In the numerous species with a long 



* ' Origin and Metamorphoses of Insects,' p. 67. 



