Integument Modifications: Additives
A Central American leafcutter ant. Image courtesy of Alex Wild Photography with permission.
Metals for hardness
Cuticle forms the anchor points for muscles. This allows legs and jaws to do their work. Cuticle can also be hardened and stiffened itself, as in ‘tools’ such as mandibles, claw tips and drilling ovipositors. In the cuticle of these tools we often see incorporation of metals such as zinc, manganese and sometimes, iron. These transition metals always co-occur with halogens such as chlorine and, or bromine. It is thought that these metals and halogens form part of proteins which help make the cuticle especially hard.
For example, some termites with zinc in their mandibles have cutting edges 20% harder than termites without the metals.
Leafcutter ants also incorporate zinc into their mandibles and only start cutting leaves when the full complement of metal has been reached.
Insects also show true mineralization of the cuticle.
Muscid flies (Musca domestica and Musca autumnalis) have a pupal case armoured with calcium. This appears to reduce parasitism presumably because wasps cannot easily drill through the outer casing to lay eggs within.
Calcium has also been found in the cutting edges of some parasitic wasp ovipositors, as crystals of calcium carbonate; in the body cuticle of some beetles, and as amorphous calcium phosphate in the cuticle of the soil-burrowing predatory larvae of the fly, Exeretonevra angustifrons (below). These are examples of true mineralization, like we see in the crab carapace.
Calcium particles on the cuticle of an Exeretonevra larva. Image: B. Cribb
The elastic cuticle protein, resilin
One form of insect cuticle is made up entirely of a protein called resilin. This protein is highly hydrated which means it contains water.
It has been isolated and manufactured in the laboratory and is the most efficient elastic protein known to mankind.
Found in and associated with joints and tendons of insects and in specialized patched which undergo repeated compression and stretching, it is the equivalent of an insect spring. The material can be bent and stretched for the life of the insect without wearing out. Because it shows a bright blue fluorescence under UV light it is easy to find. Patches are seen under the wings of dragonflies and in the legs of fleas and leafhoppers.
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(Above) A resilin cord attaches a flight muscle tendon of a dragonfly to the cuticle of the thorax. It is viewed under the microscope illuminated with natural white light (top) and with ultraviolet light that produces blue fluorescence (bottom). Image: D Merritt |
(Above) An artificial resilin cord made from the Drosophila resilin protein sequence tied into a knot and viewed with ultraviolet light (above) and white light (below). Image: D Merritt |
A cat flea showing the region where the pleural arch has a thick pad of resilin that stores energy for the jump. The pleural arch (right) was imaged under fluorescence microscopy to show the resilin pad (blue) and under transmitted light microscopy to show the brown, sclerotised cuticle around the resilin pad. Images: D. Merritt
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