by Paul Donovan
Like so many people who start out keeping reptiles, I also migrated towards keeping invertebrates as well. The two seem to go hand in hand. I think the more simple animals are, the more interesting they are. And this, for me, is certainly true with insects. Just as astronomy throws up incomprehensible numbers in terms of how many stars there are (I read somewhere there are more stars in the universe than there are grains of sand on the earth [who counted them?]), so do insects.
No one knows how many species of insects there are, but somewhere approaching one million have been described so far. However, this may just be the tip of a very large iceberg because some authorities speculate the total number could reach 30, if not 50, million species! If that were not staggering enough, some scientists estimate that, at any one time, there could be 10 quintillion (10,000,000,000,000,000,000) insects alive, meaning they are by far the largest biomass of any terrestrial animal type. That is just a mind boggling number, but I will give you an even more staggering one in a minute.
There are many reasons why there are so many insect numbers, but by and large reproduction is at the root of it. I can’t think of any insect which has just a single offspring at a time. Most have multitudes of young, and some churn them out like they are doing piece work at a factory. Termites are a great example. The queen is nothing more than a baby making machine. Some have been reported to lay an egg every two seconds throughout their life. That’s 43,200 eggs each and every day, amounting to 15,768,000 per year. Is it any wonder termite colonies are so huge?
Eggs
Most insects reproduce sexually, that is they require sperm from a male to fertilise the egg of a female. This is usually achieved by some form of internal transfer with the aid of specialised copulatory organs usually at the rear of the insect’s body. Some insects, such as stick insects can reproduce asexually. Asexual reproduction is the ability of a female to produce eggs/young without the need for a male. This form of reproduction is commonly known as parthenogenesis. I will talk more about this later in this piece.
By and large most insects are oviparous, that is when the eggs are laid outside the female’s body. The egg is surrounded by a tough protective shell called a chorion from which the fully formed young must break free. There are a number of ways in which this can be achieved. The egg may have a weak point, the embryo can secrete an enzyme which softens the chorion or, in some species, the young have a specialised egg tooth to tear it. In almost all cases, hatching from the chorion is aided by the embryo taking in liquid or air and swelling, so increasing pressure on the chorion causes it to split.
Insect eggs can vary in size, shape and number depending on the insect species. Some insects will lay them in small clusters on the underside of a leaf, while others sprinkle them over the ground.
Two phases
With such a diversity of insect species, it should not come as a surprise to learn that the mode of reproduction exhibited, follows innumerable courses which have evolved to strengthen the challenges insects face in the environments they live in, and ensure their survival. Before we look at the different types of reproduction, let’s briefly look at the two general phases insects are categorised in; ‘in-complete metamorphosis’ and ‘complete metamorphosis’.
In-complete metamorphosis’ (Hemimetabolous) is considered a primitive mode of reproduction, while ‘complete metamorphosis’ (Holometabolous) is a more advanced form. Whether incomplete or complete metamorphosis, they do both share two characteristics in common, and that is once the adult stage has been reached, with very few exceptions (bristletails and fish-moths) no further growth through moulting of the skin takes place and only the adult form can reproduce.
Incomplete metamorphosis
In-complete metamorphosis involves three stages of development. This reproductive cycle is typical of grasshoppers, locusts, mantids, cockroaches, etc. in which there is an egg stage, nymph stage and an adult form. When they hatch, the nymphs resemble miniature adults; they have mouthparts, compound eyes and, in many cases, feed on the same type of food source as the adults, but are wingless.
With each successive moult, called an instar, the nymph grows in size and increasingly becomes more adult-like. The major transformation occurs during the final moult with the development of wings and sexual organs. Though pretty uniform through the various insect species, there are a few exceptions to this rule; dragonflies and damselflies for example have an aquatic nymphal stage that differs appreciably from the adult form.
Complete metamorphosis
Unlike incomplete metamorphosis, complete metamorphosis involves a four-stage reproductive cycle, and is typical of insects such butterflies, moths, beetles, flies, etc. The female will lay her eggs in a variety of locations (usually on a host food plant), which then hatch into larvae; in flies these are commonly called ‘maggots’, and in moths and butterflies ‘caterpillars’. The larvae are little more than cylindrical feeding machines with chewing or sucking mouthparts at one end and an anus at the other. They may have small eyes, or no eyes at all, several pairs of appendages for movement, and are often brightly coloured or covered in hairs to ward off potential predators.
To achieve growth, the larva undergoes several moults. When it reaches the final moult (usually following 4 to 8 moults depending on the species), the larva ‘turns’ into a pupa commonly called a ‘chrysalis’.
Beneath the tough outer skin, a great transformation is taking place. Tissue is being reabsorbed and reassigned to ‘new’ purposes; wings are beginning to develop, as are complex compound eyes, strong legs, reproductive organs and the digestive system. The pupal stage may exist for several days, as is the case with flies, or months in the case of butterflies and moths; the pupal stage of many moths and butterflies may actually over winter and hatch the following summer when food is in abundance. Only when conditions are right, does the final stage of pupation into an adult take place.
More often than not, the larvae of Holometabolous insects feed on a completely different food source than the adults, and may only have a short time frame where a particular plant source is available, as is the case with the Mopane worm.
Amongst the Holometabolous insects, occurs one very interesting group of insects called bagworms. What separates these moth-like insects, from other insects, is that the newly hatched larvae construct a nest in which the female spends her entire life in the larval form, while the male pupates into a moth-like insect that lacks mouthparts and is unable to feed. His sole purpose is to locate a female and mate her. Once mated, the female lays her eggs and dies.
Now that we understand there are two development phases insects are categorised under, lets now turn our attention to the type of reproduction insects undertake to propagate.
Oviparity – By and large, the greater majority of insects reproduce via the laying of eggs. The number of eggs a female will lay can vary from a handful to several hundred. Once the egg has been laid, embryonic development occurs within the shell via nourishment derived from a yolk.
Viviparity – This is essentially what we know as live birth. The fertilised egg develops inside the female where the embryo derives nourishment directly from a placenta, or placenta-like structure. The young are born fully formed and fully independent. Of course, if it were any other group of animal, we could leave things at that, but insects being insects, things are never quite that simple, as four types of viviparity have been observed.
(i) Ovoviviparity – This is a mode of reproduction, in which the fertilised eggs are retained within the mothers reproductive tract in an ootheca up until the point where they are ready to hatch. Typical species which exhibit Ovoviviparity include a number of beetles, flesh flies, and some cockroaches. An ootheca with well-developed embryos inside will protrude from the rear of the cockroach’s abdomen.
(ii) Pseudo-Placental viviparity – The female retains an egg in the ovariole which has very little, or no yolk, up until the point of hatching. The embryos derive their nutrients from a placenta-like tissue. Aphids are typical of this form of reproduction.
(iii) Haemocoelous viviparity – A specialised form of viviparity in which the embryos feed on the females haemolymph (the equivalent of blood in most invertebrates) where the nutrients are taken up through osmosis. Some gall midges reproduce this way.
(v) Adenotrophic viviparity – The larvae are poorly developed when they hatch inside the uterus, and feed on a secretion (milk) the uterus exudes. The larva is born fully grown, and immediately pupates. This type of birth is widely seen in flies such as the Tsetse fly and bat flies.
Parthenogenesis
One of the most remarkable forms of reproduction, is the ability of a female to give birth without the intervention of a male; so-called ‘virgin birth’. The female gamete (egg) develops into a fully formed individual without the need for fertilisation with a male gamete (sperm). It is typical in species without sex chromosomes such as wasps, bees, ants. Stick insects, some reptiles and fish are also capable of reproducing using parthenogenesis.
Parthenogenesis is sometimes referred to as a form of ‘sexual reproduction’, but this is a rather ambiguous term, for in fact it should be more correctly called an ‘incomplete form of reproduction’, as it involves the production, initiation and growth of a female egg which is a dedicated gamete.
There are a number of different types of parthenogenesis, the two most common of which are facultative parthenogenesis and cyclical parthenogenesis. Facultative parthenogenesis is the ability of a particular species to ‘hop’ between sexual and asexual reproduction. These species, such as Orders Coleoptera (beetles), Hymenoptera (bees and wasps), Thysanoptera (Thrip’s), and Hemiptera (true bugs) can produce eggs which are capable of being fertilised with or without a male. Depending on the environment and other factors, unfertilised eggs will develop into one sex, and fertilised eggs into another.
Cyclical parthenogenesis, on the other hand, is seen in some insect species whereby parthenogenetic generations are alternated with fertilised generations. Those species which can do this, produce some eggs which can be fertilised, while others which cannot. Furthermore, those species also have the ability to determine whether an egg is laid parthenogenetically or fertilised and, at what time.
Parthenogenesis probably evolved as a survival strategy, as it ensures the continuation of the species can still take place in particular environments, or where males are uncommon.
Complex life-cycles
The reproductive life cycle of aphids is a complicated affair and unlike that of any other insect species. Reproduction follows two courses, depending on the time of year. During the summer months, aphids produce only female offspring that in turn only produce females. Come the autumn, females give birth to males, which mate and then produce large eggs. These eggs are the new generation for the following summer, and can survive harsh winters, before hatching out as wingless females.
An aphid infestation usually arises from a small number of winged adults that arrive at the plant and then begin to feed on it. If it shows to be a viable host, these ‘scouts’ will deposit a dozen or so wingless young on the tender tissue before departing to search out further host plants. The immature aphids left behind begin to feed on the sap and grow. Within about seven to ten days they then become viable adults ready to give birth to live young. As almost all the adults are females, and each capable of producing upwards of 60 offspring, the colony soon increases in size. This is compounded even more by the fact that this cycle is repeated several times leading to a massive population explosion. These dozen or so females who first arrived at the host plant, have the potential of producing several thousand offspring within a few weeks. The overburdening numbers, as they jostle for position, quickly create winged forms that take to the air in search of new host plants, and the cycle repeats itself all over again.
Tail-end Oh, I almost forgot, I promised to give you an even more staggering number, didn’t I? Well, here it is. If every descendent from a single pair of House flies, Muscadomestica, survived, in a five month period, there would be a monumental 190 quintillion individuals. Thank goodness there’s a lot of hungry predators out there!
