Wing Venation and Flight
Wing Venation
Despite their inert appearance, the wings of insects are living evaginations of the meso- and metathoricic body wall. In this sense, and quite literally during wing development, the dorsal and ventral walls are structural duplicates of the body wall and contain between them blood, tracheae, and nerves. The definitive wings of the adult insect are of course modified by union of the two walls over much of their area so that the original condition is retained only along the veins. The latter are essentially remnants of the body hemocoel. Even here the inner layers of the original body wall, the epidermis, and basement membrane, are sometimes much reduced or absent and the cuticle is thickened. The permanently appressed areas between the veins, i.e., the areas of the wing membrane, appear to be entirely cuticular. However, tracheae and nerves pass through some of them and it seems likely that the union of the two walls is not always complete and may permit a slow percolation of hemolymph (“blood”). Wings become dry and brittle in areas where blood circulation has ceased, and it seems evident that some exchange of materials is necessary in the wing membrane to maintain the resilience of the wing. In the main, however, tracheae and nerves lie within the wing veins and it is here also that an active circulation of blood occurs.
The number and arrangement of the wing veins is of great value in identification, particularly in those groups of insects that have membranous wings. There is a great deal of variation in the wing venation of different insects, but it is possible to homologize the veins and to use a system of terminology applicable to almost all insects. This system is called the Comstock-Needham system and is outlined below.
The longitudinal veins and most of the cross-veins bear names. These (with their abbreviations in parentheses) are given below together with their corresponding location in the wing.
Longitudinal Veins
The longitudinal veins vary somewhat in their method of branching in different insects, but the basic or hypothetical primitive arrangement is as follows:
Costa (C) -an unbranched vein that usually forms the anterior (or costal) margin of
the wing
Subcosta (Sc) - is forked distally. (The branches of the longitudinal veins are numbered
from anterior to posterior around the wing by means of subscript
numerals). The two branches of the subcosta are designated Sc1 and Sc2.
Radius (R) - gives off a posterior branch, the Radial Sector (Rs), usually near the base
of the wing; the anterior branch of the radius is R1. The Radial Sector
(Rs), the posterior branch of the radius, forks twice, with four branches
reaching the wing margin, respectively R2, R4, and R5.
Medius (M) - forks twice, four branches reaching wing margin, M1, M2, M3 and M4.
Cubitus (Cu) - according to the Comstock-Needham System it forks once, the two
branches being Cu1 and Cu2; according to some other authorities, Cu1 forks
again distally, the two branches being Cu1a and Cu1b.
Anal Veins (A) - are typically unbranched and are designed from anterior to posterior as first
anal (1A), second anal (2A), and third anal (3A).
Cross Veins
The cross veins are named according to their location in the wing or the longitudinal veins they connect. Refer to Fig. 3-10 as you read the descriptions below.
humeral (h) - located near the base of the wing, between the costa and the subcosta.
radial (r) - connects R1 and the anterior branch of the radial sector.
sectorial (S) - connects R3 and R4.
radio-medial (r-m) - connects the posterior branch of the radius and the anterior branch of the
media.
medial (m) - connects M2 and M3.
medio cubital (m-cu) - connects the posterior branch of the medius and the anterior branch of
the cubitus.
cubito-anal (cu-a) - connects the posterior branch of the cubitus and the first anal vein.
The wing venation of any particular insect may differ from the basic arrangement just described in that it may have fewer veins or more veins. If the venation is reduced, either one or more veins are lacking, two ore more veins are fused, or one or more veins fails to branch. A fused or unbranched vein is named on the basis of its component parts; for example, the anterior branch of the radial sector is R2+3, and it sometimes fails to branch. Other similar veins are R4+5, M1+2, and Cu +2A. Extra veins may either extra cross –veins or extra branches of the longitudinal veins. Extra cross-veins may be designated by number (for example, first radial and second radial) if they are not too numerous, or they may have special names based on their location (for example the antenodal cross veins in the Odonata) or they may be unnamed. Extra longitudinal veins are usually additional branches of the principal veins, called accessory veins; such veins, if they are not too numerous and are constant in number and position, are named after the vein from which they branch (a single accessory vein branching from M1 for example, is designated as M1.2). If there are two or more accessory veins branching from M1 (or any other principal vein) they are usually simple called M1 accessories. Other types of extra longitudinal veins may have special names, such as the intercalary veins of mayflies and the supplements of dragonflies.
Wing Cells
The spaces in the wing between the veins are called cells. Cells may be open (extending to the wing margin) or closed (completely surrounded by veins). The cells are named according to the longitudinal vein on the anterior side of the cell (refer to fig. 1 as you read the descriptions below); for example, the open cell between R2 and R3 is the R2 cell. The cells at the base of the wing are usually named after the basal or unbranched part of the longitudinal vein on the anterior side of the cell; for example, the cells R, M, and Cu. Where two cells separated by a cross-vein would ordinarily have the same name, they are individually designed by number; for example, the medial cross-vein divides the M2 cell into two cells, the basal one which is designated as the first M 2 cell and the distal one as the second M2 cell. (Where a cell is bordered anteriorly by a fused vein (for example, R2+3)) It is named after the posterior component of that fused vein (cell R3). In some insects certain cells may have special names; for example, the triangles of the dragonfly wing.
Flight
The wings of insects while evaginations of the body wall do not function like a vertebrate wing. There are no muscles located within the wing, therefore wings are moved externally. The modern insect orders to this by raising and depressing the tergum or notum. The wings are attached to the notum and are essentially tipped up and down using the pleuron or side of the thorax as a fulcrum. This movement is accomplished by two pairs of muscles. A dorsal-ventral pair that depress the notum and raise the wing (see diagram), and a longitudinal pair that pull the ends of the thorax which raises the middle and depresses the wing.
Using a honey bee worker examine the wings and thorax. First, notice that the front and hind wings are usually together. This is accomplished by a row of hooks on the hind wing that engage the forewing. These hooks are called hamuli. Next, look at the base of the wing for a set of small sclerites used in connecting the wing so that it can be folded over the abdomen when at rest.
Then, using a pin or sharp forceps, open up the dorsal thorax to see the muscles used in flight. These are tightly packed so it is a little difficult to recognize the discrete units.