A Functional System of Adaptive Dispersal by Flight
About: This article is published in Annual Review of Entomology.The article was published on 1966-01-01 and is currently open access. It has received 70 citation(s) till now. The article focuses on the topic(s): Biological dispersal.
Summary (2 min read)
- By C. G. JOHNSON Rothamsted Experimental Station, Harpenden, Herts., England Insect dispersal is a confused subject and lacks system.
- Different people have seen migration and dispersal as distinct, processes, twice reviewed in these volumes.
- Therefore if a general, valid, and functional system for all of these movements can be made; it may show up effects of incidental and accidental movements in clearer perspective (I suggest that these are small by comparison with those of adaptive move ments) .
PERSPECTIVES OF THE SYSTEM
- It is often assumed that males and females migrate similarly.
- Among the latter, some even hibernate where they become adult, dis persing only afterwards [Conophthorus coniperda (71)].
- Little seems to be known about relative distances travelled by many insects before and after ovipo sition, or between egg laying by the same individuals; but the emigration after pupal emergence (often characterized by a relatively prolonged and undistracted flight), is often the most noticeable with recognized migrants.
PLACE OF ORIGIN
- There are two fundamentally different types of "populated place " (4) from which dispersal occurs adaptively.
- Most important is the site where new adults develop.
- These changes, and those caused by the seasons affect the insects' ontogeny and apparently determine whether or not, or in what proportions, adults will become migrants, and hence the ex tent and amount of dispersal.
- If Type II movements are migratory in the senses defined earlier (and this is debatable), feeding and oogenesis sites are among these.
- There are a few observations of the beginnings of "classical" migrations especially of butterflies and dragonflies; rather more exist for migratory in sects in the wider sense, including wind-borne dispersants formerly consid ered to be "passive.".
- But, of the vast majority of even well known and com mon species, nothing is known about exodus behaviour though it is very im portant for dispersal.
- The prime function of such flights is departure, sometimes for long distances, as soon as the insects become fully flight-mature.
- There are more references it seems to the positive act of exodus (few though these are) than to others showing that adults do not leave, and it would be instructive to search out the latter.
- "Flight activity" is a universal term among entomologists.
- Though these changes are well recognized as long-term trends over several days and even corrected for (169) , their rapidity from hour to hour (when they can simulate the effects of individual responses as far as aerial density change is concerned) is not so well recognized.
- The duration and progress of this Hight away from the birthplace is adapted to the needs of particular species, varying from a few yards [ter mites, white fly (92)] to thousands of miles (locusts; monarch butterflies) can be resolved into four components: fuel supply ; duration of single flights; and duration of period when such flights are repeatable; and orientation.
- Kennedy et al. (91) inferred that aphids flying near the ground in the "alighting flight" re sponded to both the bright, short wavelengths of the sky and to the dimmer long wavelengths from the green vegetation and were kept in an uneasy bal ance between them, rather than that they became negatively phototactic.
- The common orientation of butterflies, migrating within the boundary layer, is also apparently established visually at exodus ( 1 10) .
- Many insects disperse from the birthplace by flying within their bound ary layer and seem to control their track in a more or less linear way ap parently for relatively long distances against or across the wind.
- Adaptive displacement per se depends on the following factors; flight speed, duration and frequency of single undistracted flights, the duration of the "migratory" period within which single long flights are made, the need and opportunity to "refuel, " orientation into or out of the "boundary layer, " and the state of the atmosphere.
- -A complete displacement flight often occurs adaptively both within and above the boundary layer.
- The ver tical density profile increases in height and decreases in steepness toward mid-day as more i nsects are discharged, and subsides, with the flight rhythm at exodus, to nothing by evening (81, 85) .
THE FIRST HYPOTHESIS
- The following findings support this hypothesis: Orthoptera migrated when plants dried up and after rain ( 133, 157) .
- Flight in solitary locusts prob ably was a response to humidity changes (37) .
- L. cerealium flew en masse when grasses dried up, rather in contrast to normal, prereproductive mass flights (99) .
- Leafhopper populations moved from cut vegetation (103) .
- No doubt hunger often causes insects to move but perhaps more to mix than to disperse, for starved insects cannot migrate far (74) .
THE ONTOGENETIC HYPOTHESIS
- This hypothesis applies, so far, only to females.
- In large populations of A . monuste, settling thresholds are so high that the insects continue to fly all night (dark ness usually suppresses flight) and into the second day, scarcely stopping more than a few seconds to feed, which prolonged flight makes necessary ; this behaviour is associated with delayed ovarial development ( 1 10) .
- Johnson, .c. G. A basis for a general system of insect migration and dis persal by flight.
- Smith sonian Inst. Misc. Collections, 137, 263-86 (1959) Roer, H. Experimentelle Untersuch ungen zum Migrationsverhaltl�n des Kleiner Fuchs (Aglais urticae L.) Beitr.
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