Insect development

Incomplete metamorphosis of the German cockroach

Fig. 3.4. The German cockroach has incomplete metamorphosis. This means that its offspring will gradually come to resemble their parents as they grow.

Most insects lay eggs. Some insects like cockroaches and crickets have offspring that looks a lot like the grown insects. However, they do not have wings, their genitals are not yet developed and they often have a different body shape and colour compared to the grown insects. Insects go through a number of development stages and will slowly begin to look like the adult insect. This kind of development is called incomplete metamorphosis.
In other insect groups, we find complete metamorphosis. Complete metamorphosis is seen within flies, beetles and butterflies. The complete metamorphosis goes through several larval phases, which do not have any resemblance to the adult insect.

The development towards maturity takes place in a special phase, the pupa phase that is. When the adult insect comes out of the cocoon it does not grow anymore.
Tiny cockroaches may be cockroaches, which are not yet adults, while small flies flying around, are not the offspring of bigger flies.

House fly complete metmorphosis

Fig. 3.5. The common house fly has complete metamorphosis in which the eggs become larvae. The larvae grow large and become pupae, in which metamorphosis into the adult insect takes place.

Insect development and metamorphosis are controlled by hormones. The hormones are formed in special glands on the lower side of the brain and in the prothorax. The hormones are distributed with the blood. It is a particular moult-inhibiting hormone that starts the moulting. Although it is formed in glands in the prothorax, it has been shown that hormone production in these glands is controlled by a hormone produced by glands in the brain. Besides these two hormones, the so-called juvenile hormone has a crucial role in moulting. It is secreted by a pendant on the brain and it is the amount of juvenile hormone in the blood that determines what phase a larva enters the next time it sheds its skin.

In young larvae, the juvenile hormone is present in large quantities, but hormone production decreases as the larvae grow older and when the hormone is below a certain value, the larvae begin to go into the pupal stage.
Knowledge of these processes is of great practical value. Synthetic-made insect hormones may in some cases be used in pest control without being dangerous to humans. Sprayed on insect habitats or in tiny amounts mixed in baits, synthetic hormones may mess up the vital processes so that insect development is impaired or the insects simply die.

Life cycle

In Denmark, there is one generation of harvest mites a year. The mites overwinter in the nymphal stage and evolve into adults in the early summer. The adult mites lay eggs in the ground. The larvae hatch in late July – early August. The active larvae are six-legged. They are 0.2-0.3 mm long, but because of their red or reddish-yellow color, they can easily be seen. When a larva has had a blood meal, it has almost doubled in size. It lets go of the host and fall to the ground. It finds a hiding place where it can molt and become a free-living nymph.

The older stages of the harvest mites are almost never seen. They hunt other small animals and are without particular interest to us. Adult harvest mites are red and approximately 2 mm long and 1 mm wide. On bare peat soil, big, bright red mites are often seen. These are is the so-called velvet mites. They are related to the harvest mite, but are harmless to humans.

Biology

Red mites are nocturnal. During the day they hide in cracks and crevices of birds’ nests. With caged birds, they often spend the day hiding where the perches are attached to the cage. The eggs are laid in the hiding places and the mites hide in them while molting. A female mite lays 10 eggs at a time. Each portion of eggs requires a blood meal. From the eggs, small, six-legged larvae hatch. These larvae do not feed because their mouth parts are too short to pierce the skin of a host. After the first molting they become eight-legged nymphs that suck blood, but that do not reproduce. Two molts later they become adult mites, which can lay eggs. The development from egg to egg takes about 10 days. Adult mites can starve for 4-5 months.

An unusual family

Four males on a female mite's abdomen

Fig. 85. Four males crawling around on the abdomen of a fertilized female (Rack)

When a water mite – one of the 2mm big ones – is spotted in the microscope, you will see up to several mites at once.

There are both male and female water mites. The fertilized female mite finds an insect larva (or, in the lack of a better host, a human), which she latches onto and then injects the mouth parts into the skin. After a few hours her abdomen swells up and she is sought out by one or more males. The males then spend the rest of their short lives to crawling on large abdomen of the fertilized female. When they get hungry, they inject their mouth parts into the female and suck out nourishment. In the female’s abdomen, the next generation of water mites is developed.

You can clearly see the new mites because the female abdomen is completely transparent. The water mites are born as adults. Often the males help giving birth to the new mites. They cut a hole in the abdomen with their powerful hind legs and pull out the new females for immediate mating. Males only help at the birth of female mites. New males are on their own.

Life cycle

Egg web of the fur mite

Fig. 81. The fur mite’s egg sac is an irregular lashing of fine threads that sticks the egg to the hair. Most eggs are seen on thin hairs (17 micrometers or so) and 2-3 mm from the hair roots. The egg sacs are easily seen through a microscope. (Rack)

Fur mites lay eggs. Each female lays one or two at a time and they are placed in a distinctive web and secured to a hair close to the skin. This egg hatches into a pre-larva which is similar to a fly larva with finely speckled skin and a few protuberances on the anterior end.

The relevance of this stage is unknown, and the pre-larva does not leave the egg. Inside the pre-larva, a six-legged larva is developed. It is similar in appearance to adult fur mites. When hatching, the empty skin of the pre-larva is left inside the egg while the larva crawls out through the top of the egg. After a while, the larvae attach to a hair, the mouth opening is plugged shot and the larvae remains completely still while the next stage with eight legs develop inside.

When the next stage has developed, the dorsal side of the larval skin splits, and the next stage emerges from the old larval skin. After two more similar molts, the adult females and males are developed. Females are compactly built. Males are slightly smaller and have longer legs. Males and females mate, and the females start to lay eggs. The total development time from egg to egg is not known.

Arthropod development

Most arthropods lay eggs, but there are a few that produce live young. One of the disadvantages of having an external skeleton is that growth cannot proceed gradually, and so during their life arthropods have to moult a number of times, becoming a little larger each time. Among the insects, development is associated with the process known as metamorphosis, which can take place in two different ways. In some insects, such as bed bugs and cockroaches, the newly hatched young look like miniature versions of the adults. They go through a number of stages, separated by a series of moults. This is known as incomplete metamorphosis. Other insects, such as flies, beetles and butterflies, have complete metamorphosis, in which the young stages show no resemblance at all to the adults. Here a special immobile stage, the pupa, occurs between the larva and the adult. When the adult insect emerges from the pupal stage it ceases to grow, so there is no truth in the idea that, for example, small flies can become large flies.