Tests of Functions of Insect Metamorphosis
Analysis of Several Hypotheses Other Than Food Competition
Most insect species exhibit metamorphosis. Since the young and adults of insects often eat different foods, and may also live in different habitats, scientists have suggested several alternate hypotheses for the value of metamorphosis:
- reduce competition for food between adults and their offspring
- allow young and adults to live in entirely different environments
- division of labor - separating the growth phase from the dispersal and reproductive phase
- advertisement that the individual is ready to reproduce
- enable them to weather or escape harsh summer and winter conditions
The food competition hypothesis was examined on page 3. Here the alternates are also discussed.
The Different Habitat Hypothesis
When animals with different life stages live in different habitats (such as water and air) their young and adults use different space and foods. Thus, two features of their life history are separate. But if the two stages live in different habitats, they are almost forced to eat different foods. Thus, although living in different habitats reduces competition for both space and food, the reduction in competition for food is probably more important to the animals than reduction in competition for space is as larger animals can not frequent the same microhabitat (space) that smaller animals can.
The Division of Labor Hypothesis
Although juveniles channel their energies into growing and adults channel theirs into reproducing, this is common to almost all animals and plants. Although this mechanism appears to have a strong positive survival value to all organisms, most organisms do not segregate the two life stages by a metamorphosis. Since metamorphosis is not necessary to provide this temporal separation of activities in most animals, it appears to be superfluous to division of labor for insects.
The Advertisement of Reproductive Readiness Hypothesis
Most animals provide an indication that they are ready to reproduce. This advertisement uses a wide variety of mechanisms. Birds advertise reproductive readiness through plumage and behavioral changes, reptiles and fish change color and behavior, mammals release pheromones and exhibit behavioral changes. Amphibians exhibit behavioral changes. Insects and crustaceans undergo metamorphosis, release pheromones, and exhibit behavioral changes. In most situations the pheromones and behavioral changes are sufficient for insects to attract mates. Again, the metamorphosis takes place before these changes, but seems to be superfluous to reproduction.
In support of the extraneous nature of metamorphosis for attracting mates, the females of a desert bee (Habropoda species) spend several weeks as pupae under the sand. During this time, the males of the species ignore them. Shortly before metamorphosing to adults, and while still buried under the sand, each female pupa releases a pheromone that indicates she will soon metamorphose. Males rapidly congregate around the pupa – often coming from hundreds of meters. Immediately upon leaving the pupal shell and crawling out of the ground, and before her adult skin has hardened, the female is completely covered by males – all attempting to copulate with her. Many males succeed in inseminating her. Once the female has been inseminated, she ceases producing the attractant pheromone and the males leave and she is able to expand her wings and take on the role of a flying insect. In this instance, metamorphosis is a hindrance because if the female is not inseminated early enough, the males clambering over her soft, newly molted body would destroy her tender, undeveloped wings rendering her incapable of ever flying and providing food for her young.
The Weathering or Escaping Harsh Conditions Hypothesis
Most insects that overwinter or survive harsh droughts do so during a particular life stage: egg (mosquito and some butterflies), larva (beetles, dragonflies, some butterflies and moths), pupae (some beetles, flies, bees and wasps, butterflies and moths, or adult (flies, bees and wasps, a few butterflies). In these insects, the particular life stage is uniquely adapted to withstand the rigors of drought, heat, and/or cold and previous or later stages would die if exposed to the same conditions.
All insects that migrate to escape harsh winter (monarch and other butterflies, dragonflies and damselflies) or summer (some butterflies, dragonflies) conditions do so as adults. Although the larvae of most northern dragonflies and damselflies and pupae of most butterflies can survive winter and summer extremes, the adults of most are frost sensitive and would die if they did not migrate.
Metamorphosis is essential for both activities: surviving and migrating away from harsh conditions.
Thus, the two primary functions of metamorphosis appear to be:
- relieving competition between young and adults
- surviving or migrating away from harsh weather conditions
The tests of other hypotheses suggests metamorphosis is superfluous or at best only incidental to the activity.
This article is composed of four pages:
Page 1: The control of metamorphosis.
Page 2: Types of metamorphosis.
Page 3: Possible functions of metamorphosis.
Page 4: The most probable functions of metamorphosis.
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