OF THE ACA.RIDjE TO THE AEACHNIDA. 287 



new segments in front of the anal segment is in process of being 

 lost, the segments being marked off nearly simultaneously on a 

 germinal disc. 



The absence of the seven abdominal Arachnidan segments from 

 the Acaridse is not a true ease of excalation, because the method 

 of development is still sufficiently axial to lead us to believe that 

 when the Acaridae were specialized this method of development 

 was still the rule. These seven missing segments were therefore 

 only excalatedin the sense that the development of new segments 

 ceased before reaching them. 



This method of explaining the origin of the Acarids leads us 

 almost necessarily to assume that the Acarids in each region were 

 the fixed larval forms of the Araneids of that region — fixed, that 

 is, at the time when the Araneids themselves had become spe- 

 cialized for their different habits of life. 



It seems to me that the young of Araneids infesting trees or 

 bushes would be very likely to find it advantageous to remain 

 small and inconspicuous. The juices of the jilant afford an abun- 

 dant supply of nourishment always at hand, while their small 

 size would render them comparatively safe, at least from the 

 larger animals which prey on Spiders. Their small size, on 

 the other hand, would make them very liable to be devoured 

 by Ants, against whom each species has a special method of pro- 

 tection. Tetranyclius tiliarum seems to live under a web which 

 it spins ; other Mites form galls, probably burrowing into the 

 leaf originally to take refuge from the Ants. The Oribatidae can 

 afford to wander about freely, as, apparently after trying various 

 disguises*, they have provided themselves with a thick chitinous 

 shell, like a suit of armour. Others have even succeeded in 

 making themselves the honoured guests, or valued slaves, of their 

 foes t. 



I hope in a subsequent communication to give an account of 

 the minute anatomy of Tetranychus tiliarum. 



Before concluding this attempt to show that the Acaridse may 

 be explained as Arachnids (probably Araneids) fixed in a larval 

 condition, there are two points ot considerable interest which 

 cannot be ignored in any discussion as to their morphology. 



* See the beautiful disguises of some of the larvtc iu Michael's ' Monograph 

 of the Oribatidae,' Eay Society. 



t Michael, vide abstract in 'Nature,' vol. xlv. p. 16-1. 

 LINIf. JOUElSr. — ZUOLOaT, VOL. XXIV. 22 



