354 Transactions of the Society. 



W. M. Wheeler (1901, 1907) found in the gaster of the Texan ant 

 Phcidolc cmiitmitata a worm which he considered to belonsr to the 

 genus Mermis, with which we are here concerned. He concluded 

 that tlie worm lay in the crop of the worker ant, whose fat body 

 and reproductive organs seemed to have disappeared entirely. The 

 worms were 50 mm. in length, ten times that of the host, and one 

 host contained two worms. He says ("Ants," p. 420): — 



" They enter the larva and appaiently by unduly stimulating 

 its appetite cause it to be fed excessively, so that it becomes 

 unusually large at the time of pupation, and produces a gigantic 

 worker form, wdth ocelli." Tiiis large form of worker he called a 

 " mermithergate." It is not clear how it was ascertained that the 

 worm actually entered the larva. Though it is fairly certain that 

 the parasite enters its host in the larval stage, the results of my 

 experiments so far do not sliow how and at what period this takes 

 place. Wheeler goes on to say that the parasitized ants were in a 

 constant state of hunger owing to the presence of the parasite. 

 This I have also noticed in the case of Lasius. Emery had seen 

 these mermithergates in 1890, but without realizing to what their 

 increased size was due until later, and he then also recorded their 

 presence in several other neotropical ants of different genera, and 

 concluded that the worm must enter the larva. 



Mrazek (1908) appears to have been the first to write in any 

 detail of the parasitization of Lasius by Mermis in Europe, though 

 1 found the parasitized ants in 1898, and recorded them as 

 brachypterous forms (1910). Mrazek showed that the wings of 

 the host (in this case Lasius alienus), which are the most prominent 

 secondary sexual characters in most female ants, only developed 

 to about one-quarter or one-third of their normal size. Later, in 

 1910, Wheeler found small-winged females of L. neoniger, a closely 

 related form from Colorado, some of which contained worms from 

 53 to 55 mm. in length. 



My first discovery in England of female ants parasitized by 

 Mermis (and known as mermithogynes) was in 1898 at Oddington 

 near Oxford. Along a road bounded on each side by a deep ditch, 

 wliicli for the greater part of the year contained water, had occurred 

 a marriage flight of the common yellow meadow ant, Lasius flavus. 

 This was towards the end of August, and alate and dealate females 

 were to be seen on tlie road for some days afterwards. Amongst 

 these I picked up several short-winged forms, which some years 

 later were found on dissection to contain Alermis. Two or three 

 days later in the same place I found some mermithogynes of 

 Lasiris alienus, the species wliich formed tlie subject of Mrazek's 

 paper ten years later. Again m 1900 and 1901 I found several 

 more parasitized females of both these species in the same locality. 

 These females are readily recognized by their abnormally small 

 but otherwise perfectly formed wings. I tested their powers of 



