278 THE PANORPOID COMPLEX (Tntroduction), 



mologists who have not studied other Classes of Animals, that it 

 is not necessary to be able to trace back a structure to the 

 embryo without a break, in order to prove that it is derived 

 from an embryonic structure. Particularly does this apply to 

 appendages within the Phylum Arthropoda. Examples of this 

 are the well-known observations on the re-development of limbs 

 from latent embryonic rudiments in Crustacea, e.g., the man- 

 dibular palp in Decapod larvae, and the maxillipedes of the 

 Stomatopoda. In fact, the rudiments may appear in the embryo, 

 disappear later on, and then reappear again in the imago, as in 

 the case of the thoracic legs of the genus Chalicodoma and other 

 genera in the Order Hymenoptera. 



Thus we see that it is not sufficient proof that a structure is 

 not derived from an embryonic rudiment to show that it does 

 not develop directly from that rudiment; whereas, it is, most 

 certainly, quite sufficient proof that it is so derived, if we can 

 trace it up from the embrj^o, through the larva and pupa, with- 

 out a break, to the imago (or, if the structure in question is only 

 a larval one, then it need only be traced from the embryo to the 

 larva). 



We may take, as an example of this, Lubbock's observation 

 upon the development of the lateral gills in Chloeon, which, he 

 proved, did not begin to develop until the third instar of the 

 larva. Many entomologists have used this as proof that these 

 gills cannot be homologous with the original segmental abdominal 

 appendages, which are represented in the embryo, and then dis- 

 appear. This argument is quite fallacious, and must be care- 

 fully avoided, since it is a frequent temptation to use it. Further, 

 there is another potential fallacy in this kind of argument. 

 Chloeon is a highly specialised member of the Order Plectoptera. 

 May it not be quite possible that the break between the embry- 

 onic appendages and the formation of the gills, which is so con- 

 spicuous in this genus, may be very much reduced, if not com- 

 pletely removed, in the case of the development of the larva of 

 some much more archaic type, such as Onisciy aster, not yet 

 studied ? 



