68 Eighteenth Report State Entomologist of Minnesota — 1920 



one another; how the family Aphididae grew or developed into the 

 large, complex, and interesting form that it now presents to us. We 

 aim to trace the various lines of progress, sometimes retrogression, 

 found within the family, lines that are constantly diverging but con- 

 verge as we approach their origin. The fundamental characters are 

 therefore not structural but physiological and ontogenetic, or, in short, 

 biological, as phylogeny is essentially an extended ontogeny. Such 

 characters are not easily grasped and understood, and in most cases can 

 only be expressed as these same characters have expressed themselves 

 in the modified structural. Morphology becomes in this respect the 

 language of phylogeny. Moreover we should be careful that we ex- 

 press the spirit of this language and not allow it to become a dead and 

 artificial language. 



The Pterocommini-Aphidini line has also expressed itself in an- 

 other character easily seen and very important in placing the tribe 

 Aphidini. The sensilla, or sense hairs, on the antennae, the head, and 

 sometimes also other parts of the body, are here simple or hair-like, 

 not differing much from ordinary hairs except that they have a direct 

 nervous connection. This type is characteristic of the Pterocommini 

 as well as other tribes of generalized Aphidinae. In the Aphidini they 

 are usually short, sharply pointed, spine-like structures very charac- 

 teristic of the group. In the Calaphidini-Myzini line the sensilla are 

 enlarged apically, globate, capitate, and glandular, which is also true 

 of the Calaphidini-Macrosiphini line, tho with a distinct form of the 

 enlarged apex. The globate sensilla seem to have a wider distribution 

 and are often found in the larval forms even more conspicuous than 

 in the adult. Such characters may be looked upon by some as too 

 minute and difficult for practical purposes, but the phylogenist does not 

 consider the difficulty of a character, provided it aids him to grasp the 

 true state of affairs. We may consider ourselves fortunate in having 

 to deal with a group so rich in external characters which have not be- 

 come exhausted, rather than with one in which we have to turn to 

 internal, dissected characters, as is already required in more than one 

 of the more difficult groups of insects. 



The antennae and sensoria also show some interesting differences 

 in the two lines under consideration. In the first line they are usually 

 shorter than the body tho showing a progressive increase in length 

 from the Pterocommini and throughout the Aphidini ; the spur of the 

 terminal segment is always well developed in comparison with the short 

 condition in the Lachnini. In the second line the antennae usually 

 surpass the body in length and the spur also reaches exceptional 



