90 _ NATURAL HISTORY OF WASPS. 
thorax is divided; and some anatomists count many 
more. The details of all these parts, particularly as 
displayed in the thorax of the Hymenoptera, have a 
special interest for entomologists. Our present field 
of observation, however, is much more limited, being 
almost exclusively confined to the dorsal surface, 
and to this, too, rather in a general than in a purely 
anatomical point of view. Indeed, the appendages 
of the thorax, the legs and wings, claim more of our 
attention than the skeleton. 
The prothorax is the first division of the thorax, 
counting from before backwards. In the Hymeno- 
ptera this forms one compact piece, in which the lines 
of division of the component parts are not very 
easily distinguishable. The posterior boundary, 
however, is marked clearly enough on the dorsal 
surface, by two yellow lines which run backwards, 
diverging from behind the neck to the scale covermg 
the root of the forewings. These yellow lines are 
the only distmetive marks which this division of the 
thorax displays in our British Vespex, though in some 
foreign genera, especially in Polistes, the whole of 
the dorsal portion of the prothorax is highly coloured. 
Both in the Social and Solitary group the prothorax 
is prolonged backwards to an unusual extent, appa- 
rently to give additional space within it for the front 
attachment of the depressor muscles of the wings. 
The portion of the prothorax which appears on the 
back represents the scutellum. The morphological 
nature of the loose ring or collar which surrounds 
the neck has been the subject of much discussion, 
but it is now pretty generally agreed that it repre- 
sents the sternal portion of the thorax, and should, 
