II. 
DIPTERA. 
Au suctorial msects which in the perfect state possess only two 
membranous wings, are called Diptera, from two Greek words 
—dic, two, and 7Tepov, wing. 
The Diptera were known and scientifically described at a very 
early date. They are found often mentioned by Aristotle in his 
History of Animals; and he applied the term to the same insects 
as now constitute the order. 
The absence of the second wings, common to other insects, 
which are in this case replaced by two appendages, which have 
received the name of balancers,* because they serve to regulate the 
action of flight, constitutes the chief characteristic of the Diptera. 
Let us, however, give a glance at their other organs, which have 
more or less affinity with those which exist in other classes of 
insects, preserving, nevertheless, their own especial characteristics. 
The mouth, for instance,—suited for suction only,—is in the 
form of a trunk, and is composed of a sheath, a sucker, and two 
palpi. The antenne are generally composed of only three joints. 
The eyes—usually two in number—are very large, and sometimes 
take up nearly the whole of the head. They are both simple and 
compound. The wings are membranous, delicate, and veined ; 
the limbs long and slight. In giving the history of the principal 
types of Diptera, we shall more fully explain the formation of 
these organs. 
The Diptera, by their rapid flight, enliven both the earth and 
the air. The different species abound in every climate, and in 
* Sometimes called halteres.—Ep. 
