Apterygote Insects

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The Apterygote Insects

Diplura. The diplurans, common name two-pronged-bristletails comprise of around 600 species. The abdomen has eleven segments as seen in most insects. They are small soil living insects with two cerci projecting from the last abdominal segment. Although this order was once grouped with the Archaeognatha (Bristletails) they have several features other than the cerci which serves to distinguish the two orders apart. With the abdominal segments showing clear separation and the absence of an epiproct (see bristletails for explaination) a clear difference between the two orders is evident. It is however the mouth-parts and the position within the head capsule that forms the main difference between the two orders. The mouth-parts of diplurans are sunken to a certain extent within the insects head capsule. The mouth-parts of Archaeognatha (Bristletails) are not sunken to any extent, a characteristic also seen in all Pterygotes. Both compound eyes and ocelli are absent in these insects and the tarsi on the legs have only one segment. If examined closely, i.e. with a microscope, it is possible to see the paired appendages along the abdominal segments. The cerci in some species (Japyx solifungus) have become modified into pincer like organs and are used to trap other small soil living organisms. The vast majority feed on vegetation, living in soil and under bark, their thin exoskeleton confines them to humid environments.

Protura. The proturans are very small, under 2mm long, soil dwelling insects. They have no common name, but the name Protura means simple tail. There are approximately 200 species known. These insects have no compound eyes, ocelli, or antennae, although the front legs are held out forward in a similar stance to antennae. The abdomen has eleven segments in the adult but the young have only eight, with segments 9,10, and eleven being added through post-embryonic growth, thus there is some metamorphosis. Although post-embryonic development is not seen in any other insect, it is a common feature among the athropods. This fact indicates to some entomologists that their placement within the class Insecta is wrong. The mouth-parts of proturans are similar to the diplurans in that they are some what sunken into the head capsule. The mouth-parts are sharp and used for piecing fungal hyphae and decaying vegetation. As the forlegs are held out in front of the body to fulfil the role of the antennae only the mid and hind legs are used for walking. The one segmented tarsi end in a single claw. The order is divided into families by the structure of the appendages which occur on the first three abdominal segments only.

Axelsonia springtail showing forked jumping organ

(Springtail)Axelsonia.

Collembola.

Springtails are rarely longer than 5mm and are named after a special forked organ which enables them to jump considerable distances as an escape mechanism. The name Collembola is Greek and refers to the sticking function of the ventral tube, kolla meaning tube and embolos meaning peg. The abdomen has only six segments rather than the usual eleven. The first abdominal segment carries the ventral tube. This is formed from the appendages of that segment. The tube itself comprises of vesicles which the insect can expand by forcing haemolymph into them. The organ has a dual function, one; to regulate water balance and the other to act as a sticking device. These insects are able to climb smooth surfaces using this organ.

The third and fourth abdominal segments form the organ responsible for the jumping mechanism. The appendages on segment four are partially fused and form the structure called the furcula, (shown above). The furcula can be folded away underneath the insects body when not in use. To retain it in its stored position segment three has appendages which again are partially fused and form the hamula, (shown above). To jump the springtail releases the furcula from the restraining hamula. Muscles pull the furcula backwards, causing it to contact the ground, the insect is then driven forwards and upwards through the air. Most of the order have this structure, but there are some species in which the furcula is only very small or even absent.

The Collembola is the largest of the Apterygote orders with between 1500 to 2000 species known at present, the discrepancy in the figures arrives from different aurthorities reporting different known numbers, it is likely to be within those quoted here. Most springtails are susceptible to desiccation and therefore live in humid environments, such as leaf litter and soil. Some however are found in the permanent snow cover of Antarctica and the Arctic, feeding on vegetation which is carried by the wind, such as pollen grains. The mouth-parts are sunken into the head capsule, as seen in both the diplurans and the proturans but rather than piecing they are biting crushing mouth-parts.

There is virtually no metamorphosis throughout a springtails development, but it does have six to eight instars in its lifespan. Some springtails live on the surface of still water on which they scavenge for detritus, while others are pests of crops feeding on clovers, peas and other crops. Sminthurus viridis also called the Lucerne Flea is an example of a crop pest species in Europe.

Archaeognatha. A bristletail large compound eyes Eyes.

A Bristletail

Bristletails form an order consisting of around 350 known species, they are susceptible to dessication and are found under bark, in soil and leaf litter or rock crevices. The name is derived from their mouth sructure which has monocondylic mandibles. This means they have only one condyle. A condyle is the name given to the attachment point of the mandible to the head capsule, most insects have two condyles, having a dicondylic mouth structure. The name Archaeognatha actually means Archaeo-beginning gnathos-jaw. These insects have two distingushing features; large compound eyes and an abdomen which can bend rapidly downwards propelling the insect into the air. They have appendages on their abdominal segments (2-9) called styles which can be moved by muscles within. These unjointed styli which are each inserted into a basal sclerite, which is thought to represent the coxa. The abdomen ends with two cerci and one median caudal filament. These insects generally lay eggs singularly at random among the leaf litter and soil. There are usually at least six instars. Some species are known to be parthenogentic.

Zygentoma.

Silverfish are common insects found in houses, mostly in association with food cupboards within kitchens. In these locations they feed on starchy material such as flour or the glue used in cartons. The body itself is dorso-ventrally flattened and covered in extremely small scales, it is these scale which give the insect its silvery sheen, hence the name. The fish part of the name may derive from the insects shape and movement, or from the fact that if you attempt to handle one it is slippery to touch, due to the scales being shed onto your fingers. The antennae are frequently longer than the body itself, and very slender. The tarsi can have two, three, or even four joints. Styli are present on most of the abdominal segments. The last abdominal segment gives rise to two cerci, which are long and jointed, plus an epiproct, commonly called a tail. The Firebrat Thermobia domestica is also found in kitchens and other warm indoor environments such as bakeries feeding on starchy foods. Although similar to silverfish it has much longer cerci and antennae.