I remember being delighted when, as an undergraduate studying zoology, I first came across the term ‘spaced out gregariousness’. This memorable phrase was coined by Professor J S Kennedy (1912-1993) and colleagues to describe organisms such as the sycamore aphid, which are gregarious – they are attracted to the presence of another aphid – but keep a certain distance between themselves. Unlike some other aphids which readers may have noticed, like the black bean aphid – which forms dense clumps – these spaced out aphids “like to be in a crowd but to have their own personal space”, to quote another aphid biologist, Professor Simon Leather (1). As we shall see, they are to a certain extent repelled by each other at a fine scale, but attracted enough to want to be as close together as they are comfortable with! (2).

How does this work? There exists around each stationary (or settled) aphid, a ‘tactile envelope’ and if any appendage of a neighbouring aphid intrudes into this space, they swing their antennae, kick their legs and sway their bodies! (2).  If all that touching gets too much for them they move away! So by a process of contact and jostling they manage to space themselves out so that they are each surrounded by a roughly circular ‘reactive tactile envelope’ – see below (and reference 2). There must be a bit of jostling and readjustment from time to time as they do need to move about the leaf and plug into new feeding sites.

Notice in the following image (within the oval) how the left antenna of one sycamore aphid (on the right) is just touching the middle right leg of the individual on the left! Presumably, this sort of touching and testing goes on all the time.

There is some leeway in the system though, because as sycamore aphid populations build up they become more densely spaced on the leaves, or at least there are more individuals close to each other (6). The spacing all seems to be done by touch rather than vision. Researchers found that if you cut off their long antennae they all shuffle up and end up closer together, as their shorter legs do not reach anything like as far as their antennae!

It is apparent from the above photograph that the aphids appear to favour one side of the leaf in this case and seem to be avoiding large – relative to their tiny size – portions of the leaf. This is probably because this unoccupied part of the leaf is unsuitable as a result of being brushed by other leaves when the wind blows. The fact that the aphids would be knocked or brushed off by the regular movement of leaves in the wind, means that the space that they can occupy on favourable leaves, like this new growth, is more restricted than might at first be supposed (4). The apparent abundance of space on the leaves is therefore a bit misleading as the sycamore aphids have to sit and extract their food in a relatively safe and sheltered micro-site, e.g. within folds in the leaf (5). The micro-climate under the leaves is another factor which may determine their distribution; subtle differences in temperature or humidity may occur at a level we large humans cannot detect.

These aphids have worked out a way of being ‘solitary and gregarious’ (2) at the same time! They like their own personal space but benefit from being in a loosely aggregated group, within reach of one another but with enough space to avoid bumping legs and antennae too many times with their neighbours. Very British you might say!

To gauge the size of these aphids, the following image shows a small group of sycamore aphids on the underside of a leaf (pointing the camera upwards) with a fly silhouetted on the other, top side.

To have a very close up look at these aphids – including their different life stages – and their predators and parasites: see the following link (7).

1. https://simonleather.wordpress.com/tag/spaced-out-gregariousness/
2. Kennedy, J. S., and L. Crawley. 1967. Spaced-out gregariousness in
   sycamore aphids Drepanosiphum platanoides (Schrank) (Hemiptera,
   Callaphididae). J. Anim. Ecol. 36:147-70.
3. Brady, John. “JS Kennedy (1912-1993): A clear thinker in behavior’s confused world.” Annual review of entomology 42.1 (1997): 1-22.
4. Dixon, A. F. G. (1969). Population Dynamics of the Sycamore Aphid Drepanosiphum Platanoides (Schr.) (Hemiptera: Aphididae): Migratory and Trivial Flight Activity.”Journal of Animal Ecology 38(3), 585-606.
5. Dixon, A., & McKay, S. (1970). Aggregation in the Sycamore Aphid Drepanosiphum platanoides (Schr.) (Hemiptera: Aphididae) and its Relevance to the Regulation of Population Growth. Journal of Animal Ecology, 39(2), 439-454.
6. Dixon, A. F. G., & Logan, M. (1972). Population density and spacing in the sycamore aphid, Drepanosiphum platanoides (Schr.), and its relevance to the regulation of population growth. The Journal of Animal Ecology,  41(3), 751-759.
7. http://influentialpoints.com/Gallery/Drepanosiphum_platanoidis_common_sycamore_aphids.htm