408 MK. J. B. GATENBY ON THE BIONOMICS, EMBRYOLOGY, 



Discussion. 



There is no doubt that the parasitism in Hymenoptera is a specialized 

 development. In such a parasitism as we see in these insects there are two 

 main facts which attract our attention : — In the first place there is the 

 altered system of respiration, and in the second there is the food and excre- 

 tion question. Non-parasitic insects are notable lor their wonderful method 

 of oxygenating their system by means of trachese, and for the large quantity 

 of food which passes through their digestive systen). In the case of internal 

 parasitic Hymenoptera there is no defecation, and in most cases the relation- 

 ships of the Malpighian tubes (if present) are altered (13). It is quite 

 obvious that were these parasitic larvse similar to other free-living forms 

 in the extent in which they got rid of excreted matter, the system of the 

 host would rapidly become poisoned. In Microgaster I have discovered a 

 new organ, the vesicle glands, which very probably takes the place of 

 Malpighian tubes, which are here quite absent as such. There is undoubtedly 

 some matter excreted by these tubes, and set free inside the hgemocoel of the 

 Porthesia caterpilhir, but the Malpighian tubes of the latter are possibly 

 quite able to cope with the extra work which would thus be put on them. 



I have not yet made a comparative study of Malpighian tubes in parasitized 

 and non-parasitized individuals, but probably it will be found that the tubes 

 in the latter are either hypertrophied or fuller of excretory crystals than 

 those of the non-parasitized caterpillars. 



It must be remembered that the parasitic larva generally feeds on the fat- 

 body of the host, and its food is therefore not likely to give rise to much 

 defecatory matter. Digestion in Microgaster consists mainly of the assimila- 

 tion of the fatty contents of the fat-body of the host. Evidently the defe- 

 catory matter remains in the gut till the anus opens later, but it must be 

 pointed out that it would be a mistake to think that there was not something 

 specialized in the digestive processes of the Microgasteridse. Attention 

 should be drawn to the fact that many predatory insects of a parasitic 

 nature live by sucking the blood and juices of other insects which they 

 capture, and such blood-sucking forms invariably defecate a good deal, 

 although their food is as pure as it seemingly could be. A careful histo- 

 logical examination of the alimentary epithelia of several non-parasitic 

 forms, and a comparison of these with that of Microgaster, leads me to 

 consider that digestion in the Microgasteridse is mainly a process of the 

 transference of the small fat drops of the host fat-body to the vacuoles in the 

 epithelium of the gut of the parasite. 



Parasitized caterpillars, towards the end of their life, become sickly, and 

 in many cases where there are bright pigment spots, these tend to be 

 discoloured ; the whole insect looks " out of condition/^ and such an 

 appearance cannot altogether be due to the starvation of the system ; rather 

 would one suppose that the accumulated effects of the excreted matter of the 



