Author: Chris Hodgson, OVA Natural Environment 

This series of short articles will discuss facts about animals and plants that are perhaps less well known but which are amazing.  I am starting the series talking about aphids (green fly, black fly, etc).   Yes, I know they are a bane to anyone growing plants, particular gardeners, but they are also among the most extraordinary animals you will meet.  They look incredibly uninteresting, but I hope I can convince you that they are not!

This first essay will describe their feeding habits etc.  Aphids face a number of problems in obtaining food.  They are plant sap-sucking insects.  The sugary sap that they feed on is the product of photosynthesis in the leaves and is transported to the growing parts of the plant down vessels known as the phloem that are located deep within the plant.  What, then do aphids use to reach these phloem vessels? 

The basic insect mouthparts consist of an upper lip and lower lip (known as the labrum and labium) with two sets of jaws in between that work laterally – the mandibles and the maxillae.  This arrangement is typical of, for instance, beetles and caterpillars.  Aphids have the same arrangement – except that the lower lip and both jaws (mandibles and maxillae) are extended into long needle-like structures, known as stylets.  Each is only a few microns in width but can be very long - those on aphids are usually about half their body length but in other closely related insects, these stylets can be much longer than the body! - typical of, for instance, insects that suck sap through tree bark.  Together these stylets form a ‘stylet bundle’, which has two very narrow ducts running through it – one duct for the saliva to run down and the other for the plant sap to run up.  So, the mandibles and maxillae form a feeding tube, and are more or less surrounded by the labrum or lower lip, which is grooved to support them.  

So - aphids have mouthparts that are long enough to reach the phloem – but how do aphids get their stylets into the plant and how do they know where the phloem is?  Each stylet is manipulated by a set of strong muscles in the head which can move each stylet independently.  By moving each in turn, the stylet bundle can be pushed into the plant – mainly between the cells.  By careful manipulation, the stylets can even be made to change direction within the plant, but they still generally lie between the cell membranes.  So far so good – BUT finding the phloem is rather like hunting for a needle in a haystack - there are no outward indications where it is in the plant.  Also, the plant has evolved defences against sap-sucking insects in the form of other structures through which the stylets cannot penetrate (sclerenchyma).  For instance, the squarish stems of broad beans have sclerenchyma running up each corner  with the phloem vessels lying beneath them.  The aphid, therefore, has to go around the sclerenchyma to get to the phloem.  Nonetheless, an aphid feeding on a broad bean stem can locate the phloem in about half and hour!  How do they mange this?  The stylets have sensory structures on their tips which can detect changes in sugar concentration.  As mentioned above, the phloem transports the sugars manufactured during photosynthesis and so the concentration of sugar within these vessels is high.  There is therefore a sugar concentration gradient from high in the phloem to low near the plants’ cuticle.  The aphids are able to detect this gradient and so manipulate the stylets to penetrate along this increasing concentration towards the phloem.  Microscope studies have shown that aphids, whilst very good at this, sometimes lose their way and have to start again.  Nonetheless, it is remarkable that they can reach the phloem so quickly.

The next problem that aphids have to solve is that plant sap is about 90+% sugar, with very little of the amino-nitrogen that the aphids actually need for growth.  This means that they have to imbibe very large amounts of plant sap in order to get enough amino-nitrogen to grow and reproduce.  How can they manage this?  Aphids have a greatly modified gut in which the posterior part (hind gut) runs alongside the anterior part (foregut).  During feeding (which is almost continuous!), the sugar is separated from the rest of the sap in the foregut and is actively pumped through its walls into the hindgut, leaving a small amount of other sap ingredients, including the amino-nitrogen in the foregut.  With an almost continuous flow of sap through the gut, enough amino-nitrogen is accumulated quite easily.  However, this active pumping of sugars through the gut wall requires a lot of energy – which, of course, is easily obtained from the excess sugar!  

A further ‘problem’ experienced by aphids is that the plant is turgid.  In other words, in order to prevent the plant from wilting, all the contents of vessels such as the phloem are under great pressure.  In fact, this high pressure probably works to the aphids’ advantage in that, once the stylet has pierced the phloem, the pressure within it pumps the sap up the stylets and into the aphids’ gut!  To control this flow, the aphids have strong sphincter muscles in their mouths.

What happens to the excess sugar?  This is passed out through the anus as a liquid known as honeydew.  The next essay will discuss the importance of honeydew (much of the economics of Greece and Turkey rely on it!).  And I will discuss how aphids defend themselves.