Like any other class of organisms, aphids have their own adaptation to feeding. This can be easily deduced from their high precision in determining host plants from non-host plants. In this class, the mouthparts of insects are adapted to the various feeding modes which range from chewing to sucking. In our case of study, aphids have mouth parts which are highly adapted to sucking from plant phloem.
Theory of the experiment
Aphids are insects that can be a nightmare to farmers since they can become a perennial pest highly reducing the returns of the farmer. They are sucking insects which pierce through the plants and suck the nutrients from the plant’s phloem tissue. There are many species of aphids but the common ones are cabbage aphids, pea aphids and onion aphids.
Closely looking at the aphids, they posses some biological factors that make them best suited for their type of feeding. One of these factors is the ability to reproduce without necessarily mating. This gives an advantage of rapid reproduction compared to if they reproduced sexually. The other biological advantage is the ability to produce both winged (alate) and wingless offspring within the same species and clone.
Several biological factors have been put forward proposing the high precision in host determination in these organisms. One of these factors is the ability to detect wavelengths of reflected light from plant leaves. It is believed that alate aphids are first attracted towards a possible host by phototropism whereby the aphids detect a certain reflected light wavelength and flies towards its source. However, this is not a precise mode of detecting a host plant amidst many potential hosts.
Upon landing on the leaf, the aphid moves on the leaf determining its texture and moving its antenna over the surface of the leaf to determine its viability as a host. The aphid then moves on to probe the leaf and determine whether the sap of the plant is of the required nutritional value. If the organism feels that the sap is a viable one, it then carries out stylet insertion into the leaf’s inner cells in the palisade layer and mesophyl layer as well. The process of stylet insertion usually takes a longer duration compared to probes which only pierce the epidermal layer of the leaf.
These organisms also appear to have a primary and secondary host. The primary host is fed on all round the year while secondary hosts are fed on only in some seasons of the year. So as to determine whether a host is primary or secondary, study the rate at which aphids leave the surface of the two specimen plants. The specimen with low rate of aphid turnover is the primary host while the one indicating a high rate is a secondary host. Chemo taxis also appears to play a significant role in aphid diet choice. In our experiment, we are to deal with how aphids determine their hosts and also consider another organism, the caterpillar used in experiment one.
Collect three Petri dishes and in each place a piece of cabbage, onion and pea leaf on a third section of each dish.
Place a filter paper at the bottom of each dish separating the specimen.
Label the dishes A, B and C. the aphids are in vials labeled A, B and C.
Take 6 aphids from each vial and place in the correspondingly labeled dish at the middle of the dish. Cover the set and leave it overnight.
Check the set up the following day and closely count the number of aphids on each specimen.
On the caterpillar experiment;
Place a piece of cabbage, soy bean and chives leaf in the arena. Place the caterpillar in the same arena and leave the set up overnight. On the following day, find out the food specimen that the caterpillar will be on.
In dish C, one aphid was found on cabbage sample, two on rose leaf sample and three on none of the samples provided. This indicated that vial C had a six of its composition of aphids as cabbage aphids, a third as rose aphids and a half as other aphids but not the aphids under study. No chive aphids were found to be present in vial C.
In the caterpillar experiment, the caterpillar was found on the soy bean leaf where it can be said to have been attracted towards.
Closely monitoring the response of these organisms, it is clear that chemo taxis played a big role in identification of possible hosts. The leaves released certain chemical substance which either attracted or repelled the aphids. Any aphid attracted towards any leaf is said to be positively chemo tactic towards that substance. In the case where the aphids did not move or feed on any of the samples provided, the chemicals produced by the food samples did not augur well with the aphids thus the plants were rejected by the aphids.
On the case of the caterpillar, chemo taxis played an important role in identification of the suitable food sample for the organism. Other samples produced chemicals that repelled the caterpillar thus it fed only on the soy bean leaf.
All potential hosts can be fed on by an organism but an organism has to find its best host. For the high precision of substrate, the organism has to depend on chemicals in the host and how they relate to the organism. If the relationship is coherent, then the organism has to find better means for further analysis of the food sample.