Mabch 5, 1909] 



SCIENCE 



385 



the imaginiform, whicli occurs in the Ameta- 

 bola (Heteroptera, Orthoptera) and is very 

 similar to the imago into which it develops, 

 the semimaginiform, which occurs in the 

 Hemimetabola (Amphibiotica, some Homop- 

 tera) and is less similar to the adult, and the 

 irue larva of the Holometabola (Hymenop- 

 tera, Diptera, Coleoptera, Lepidoptera, etc.), 

 which is succeeded by a quiescent pupal stage. 

 The organs of the various larvae are consid- 

 ered under the following heads : 



1. Larval organs that are simpler than those 

 of the imago but of neariy the same structure, 

 or such as are absent in the imago but are 

 nevertheless of a primitive character, as 

 shown by comparison with their homologues 

 in lower insects. Examples of such organs 

 are the mouth-parts and antennae of ephem- 

 erid larvae, the cerci of campodeiform larvae, 

 non-pentameric tarsi, etc. 



2. Organs that are more or less atrophied 

 or vestigial in both larva and imago, e. g., the 

 labium and maxillae of Coreihra and Chirono- 

 mtis, the larval eyes of Gorethra, etc. 



3. Organs that were first acquired by the 

 imago but subsequently transmitted to the 

 larva, or that have taken on the imaginal 

 form secondarily in the larva, like the sucking 

 mouth-parts of the Hemiptera. To this cate- 

 gory Deegener also assigns organs which ap- 

 pear in the larva as primordia of imaginal 

 structures. These he calls secondary imaginal 

 d/iscs, in contradistinction to the primary 

 imaginal discs which are represented by such 

 structures as the wing-pads of the imagini- 

 form and semimaginiform larvae. 



4. Organs that have been acquired by the 

 larva independently of the imago and are 

 either completely lacking in the latter (pedes 

 spurii, sericteries) or have been acquired by it 

 secondarily (external gills of some Perlids, 

 rectal gills of Odonata, etc.). Such struc- 

 tures are designated as provisional organs of 

 the first order. 



5. Organs common to both larva and imago 

 but developing in different directions in the 

 two instars (sucking mandibles of the larval 

 Eemerolius, Chrysopa and Dytiscus; digging 

 legs of Cicada larvae). Such structures are 

 called provisional organs of the second order. 



6. Organs that are tjrpical or primitive por- 

 tions of the insect organization but are com- 

 pletely retarded in their development during 

 larval life and remain as primordia, or imag- 

 inal discs. These are called tertiary imaginal 

 discs to distinguish them from the primary 

 and secondary imaginal discs mentioned 

 under (3). The gonads and their ducts, es- 

 pecially the latter, may be included under 

 this sixth category, but in one sense they form 

 a category by themselves, as they are not 

 specifically insect organs and as the gonads 

 sometimes mature during larval or pupal life. 



Deegener calls attention to the fact that 

 none of the organs of the imago is actually 

 lacking in the larva, but that the latter may 

 possess organs which do not occur in the 

 imago. He concludes from this that the true 

 larva " must be derived from the imago and 

 hence presupposes the existence of the imago, 

 and that therefore this is phylogenetically 

 older than the larva, but that the true larva 

 is younger phylogenetically than the imagini- 

 form young of the Epimorpha and semi- 

 maginiform young of the Hemimetabola." 

 This conclusion and Deegener's classification 

 of larval characters would appear in a some- 

 what clearer light had he not neglected to take 

 the embryo into consideration. A single ex- 

 ample will make this statement clear. It is 

 well known that certain embryonic organs, 

 such as the thoracic appendages and antennas, 

 are lacking, as appendages, in the vermiform, 

 or apod larvae of many Hymenoptera, Diptera 

 and Coleoptera, but are present again in the 

 imago. This fact alone proves that the 

 vermiform larva is an extreme coenogenetic 

 adaptation. It also throws light on another 

 matter which Deegener, Heymons, Berlese 

 and a host of other writers seem not to have 

 clearly grasped, namely, the significance of the 

 relations of the abdominal appendages of the 

 embryo insect to the so-called prolegs (pedes 

 spurii) of the larva. Deegener says of these : 



I assume that the pedes spurii do not arise 

 directly by transformation from the appendages 

 of polypod ancestors, and hence that they are not 

 phylogenetie recapitulations (any more than are 

 the tracheal gills) but are to be regarded as new 



