42 Dr. A. S. Packard, Jr's Revision of the. 



Zoological Cliaracters. 



The most useful characters for separating genera and species, are 

 those drawn from the appendages of the body, and those parts to 

 which they are inserted. In the hymenoptera as a general rule, the 

 base or insertion of the abdomen, where there is the greatest movement 

 of the parts on themselves, and the end, wherein is placed the ovipo- 

 sitor or sting, or male organs of reproduction, vary much more than 

 the middle of that region ; so in the thorax, the prothorax is more useful 

 than the meso- and meta-thorax, in being more variable, though not 

 always so. In the "propodeum" (Newman) of hymenoptera, we have 

 greater changes wrought than in any other parts of the trunk, and to 

 this part especially the observer must always recur. In the head, 

 we find greater variability as we go forward from the base. The cly- 

 peal region is of constant use, while the epicranium, and occiput espe- 

 cially, afford slight characters. In the wings, always in requisition 

 among the hymenoptera, lie characters of the first importance in genera 

 and species, but not so useful in the larger groups. Of the appen- 

 dages, those of the head and abdomen are often more useful as a gene- 

 ral rule than the thoracic — though not so available from their inacces- 

 sibility. There is thus a greater tendency to variation as we proceed 

 from the centre of the body, taken as a whole, outwards to the peri- 

 phery; the appendages vary more than the trunk, and the terminal, 

 most differentiated portions of the appendages, vary more than their 

 bases; so in considering the head, thorax and abdomen separately, the 

 variation proceeds from the middle of each region, anteriorly and pos- 

 teriorly. It is those parts most differentiated and therefore put to 

 the most constant use by the insect that vary most. The peculiar 

 habits and wants of the insect predetermine, or we would prefer to 

 say, are correlated with its peculiar structure. Thus in the social 

 bees which have to accumulate stores of honey, the brushes of hair 

 on the legs are greatly developed over those solitary species, such as 

 Halictus, which lay up slight stores in their single isolated cells; 

 while in Jornada which cuckoo-like, is parasitic on other genera such as 

 Andrena, the legs are almost naked; and in many genera of fossorial 

 hymenoptera which are carnivous, the legs are slender and entirely 

 naked. 



The Crabronidse afford, so far as we are acquainted with their habits, 

 must interesting examples of the interdependence of structure on the 

 habits of the insect. As a group, they are essentially wood-wasps, 

 making their cells in cylindrical holes in rotten wood, or enlarging nail- 



