THE ADULT DRAGONFLY 13 
found to increase progressively throughout the order, as shown by 
the following figures: 
Families Angle of humeral suture* Angle of tilt of wing 
bases** 
Aeschnidae 21 50 38 22 35 26 
Libellulidae 29 52 40 18 38 26 
Agrionidae 43 64 56 35 61 39 
Caenagrionidae 59 72 64 38 62 51 
* This is the angle that the humeral suture (the foremost of the three 
lateral sutures), viewed from the side, makes with the perpendicular. 
** This is the angle that a line drawn through the wing bases makes with 
the axis of the body. 
ABDOMEN 
HEAD THORAX 


Fig. 2. Side view of the club tail Gomphus cavillaris; sp, sp, spiracles of 
thorax; cor 1, 2 and 8, basal segments of the three pairs of legs; eps and epm, 
episternum and epimeron of mesothorax; eps’ and epm’, same of metatho- 
rax; g pl, genital plate; x, mesinfraepisternum. 
The primitive position of thoracic segments was doubtless at right 
angles to the axis of the body, and, indeed, this condition is still found 
in the early developmental stages of dragonflies. With the develop- 
ment of large wings and greatly increased wing muscles, these segments 
are slanted backwards in such a way as to throw the wings far back 
(in the Agrioninae, indeed, the front pair of wings is placed directly 
above the hind legs), while the legs are somewhat in front of their 
normal position. When we consider the resting habits of dragonflies, 
their momentary pauses and sudden flights, we appreciate the ad- 
vantage of this adaptation. The legs are thrown forward where they 
readily reach and grasp the vertical stem and the wings are shifted 
backward, and tilted, so that their cutting edges are directed obliquely 
upward, in which position a simple sculling action lifts the body 
instantly from its support. The dragonfly can thus launch itself all 
in an instant from a position of rest. 
