ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 977 



The " rods " correspond to the different forms of tapetum. In 

 Micrommata they are closely packed, perpendicular to the tapetum, 

 without being arranged in rows. Where the tapetum is funnel-shaped 

 the " rods " have a V-shaped arrangement ; they are fewer in number, 

 and not surrounded by pigment. In the other type of tapetum, the 

 "rods" are of equal length and disposed in two rows on each tape- 

 tum band. Their histological characters are briefly noted. They 

 are still the modified ends of the retinal cells, but are sometimes 

 distinctly composed of two halves. The position of the retinal nuclei, 

 and the characters of the nerve-fibres are then described. This 

 portion of the memoir closes with a brief notice of associated blood- 

 vessels and muscles. After his notice of modifications in the different 

 genera already noticed, Prof. Bertkau sums up in some general state- 

 ments and comparisons. 



Ant-like Spiders.* — Prof. P. Bertkau notes the not unfrequent 

 occurrence of mimicry among Arthropods, and directs attention to 

 the ant-like forms of some spiders. Very frequently the ant-like 

 appearance of insects is entirely superficial, disappearing on close 

 inspection. Numerous Rhynchota, and most familiarly Alydus cal- 

 caratns illustrate this ant-mimicry. Here the resemblance is chiefly 

 due to the median constriction, the dark-brown colour, the simi- 

 larity in size, and the slight difference in size between head and tail. 

 In such an instance as the beetle Clerus formicarim, the chief resem- 

 blance is that of colour. 



Among spiders, the Attidse frequently present close resemblance 

 to ants. The cephalothorax and the posterior part of the body are 

 often approximately equal. The Drassidse also exhibit frequent in- 

 stances of ant-mimicry, as, for instance, in the genera Phruolithus, and 

 especially Micaria. Among Thomisidee and Epeiridse ant-mimicry 

 seems impossible, but the Theridiadse furnish a beautiful instance in 

 Formicina mutinensis. On elms infested by Lasius and Formica, an 

 ant-like Lasseola p-ocox occurs, but as the mimicry is exhibited only 

 by the developed males, which eat but little, the resemblance must 

 be purely protective. 



Heart of Acarina.t — The Acarina have hitherto been described 

 as without hearts. In 1876 Kramer asserted the presence of a 

 pulsating heart in Gmnasus, and the discovery has been more than con- 

 firmed by the researches of Herr W. Winkler. 



a. In numerous Gamasidte Winkler was able to detect the 

 presence of a heart. This reduced organ is a short, broad, compressed 

 chamber, with two valved openings on the upper surface on either 

 side, and a delicate aorta opening freely into the body-cavity above 

 the brain. The lips of each cleft exhibit a muscle-nucleus, and four 

 pairs occur symmetrically on the upper wall of the heart. The heart 

 is moored anteriorly and posteriorly by connecting fibres, attaching 

 it to the dorsal surface. The pulsations are extremely rapid — 



* Verb. Naturh. Verein Rheinlamls, xliii. (1886) pp. 66-9. 

 t Arbeit. Zool. Inst. Univ. Wien (Clans), vii. (1886) pp. 111-8 (1 pi. and 

 1 fig.). 



Ser. 2.— Vol. VI. 3 S 



