The primitive winged insects cannot fold their wings, which in repose
are held erect over the body or straight out from the sides, as
illustrated in figs. 26 and 29. Although many types of these insects
lived in bygone ages, only two orders have survived to the present. Both
occur abundantly in Illinois.
Ephemeroptera
Mayflies
A group of insects in which the nymphs or young live in streams and
lakes; the adults are found along the edges of the streams or lakes from
which they have emerged. Mayflies are unique in that the full-grown
nymphs molt into winged insects that are not quite mature and that molt
again, usually the next day, when they emerge as fully mature adults.
The nymphs are varied in shape and have short antennae, long legs, which
are often flattened, and three tails at the end of the body. The adult
flies have very long front legs, short antennae, practically no
mouthparts, usually two pairs of wings, and two or three long tails.
When a mayfly is at rest, the wings are held together above the body.
_Hexagenia limbata_ (Serville), figs. 26 and 27, is one of the very
common Illinois mayflies and is an important factor in the food economy
of many fish.
Mayflies, formerly called Plectoptera, together with stoneflies,
caddisflies, and midges, constitute a very large portion of the life of
our lakes and streams; all four groups are important as fish food.
Odonata
Dragonflies, Damselflies
Another order in which the nymphs develop in streams, lakes, or ponds,
and in which the adults are aerial. The nymphs have short antennae, long
legs, and either a stout body with no tail, as in _Anax junius_ (Drury),
fig. 28 (dragonfly nymph), or a slender body with three large leaflike
gills projecting from the end of the body (damselfly nymph). A most
distinctive feature of this order is an extensile, highly modified lower
lip that fits like a mask over the face of a nymph. The lower lip is
hinged to extend forward and seize the small animals upon which the
nymph lives. The adults are large, often beautifully colored, as is the
_Tramea lacerata_ Hagen, fig. 29. They have chewing mouthparts and two
pairs of large wings, very finely and intricately netted with veins.
[Illustration: Fig. 26.--Ephemeroptera. _Hexagenia limbata_, the
adult form; this mayfly is also called shadfly or willowfly.
Mayflies sometimes emerge in great swarms and congregate in piles
around bridge or city lights. Actual length about 1.0 inch.]
[Illustration: Fig. 27.--Ephemeroptera. _Hexagenia limbata_, the
nymphal form of the mayfly in fig. 26; in this stage the mayfly
lives in water, emerging when full grown. Actual length about 1.0
inch.]
The order is divided into two types; the adult flies are told apart as
follows:
Body stout, wings broad at base, the front and Suborder
hind wings different in shape. Strong fliers. Anisoptera
Dragonflies
Body slender, wings narrowed at base, the front Suborder Zygoptera
and hind wings similar in shape. Weaker fliers
than dragonflies but nonetheless elusive.
Damselflies
[Illustration: Fig. 28.--Odonata. Nymph of _Anax junius_, a
dragonfly widely distributed in Illinois. Actual length of
full-grown nymph about 2.3 inches. (Drawing courtesy of C. O.
Mohr.)]
[Illustration: Fig. 29.--Odonata. _Tramea lacerata_, a dragonfly
commonly found near ponds and drainage ditches in Illinois.
Wingspread about 3.2 inches. (Drawing courtesy of C. O. Mohr.)]