ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
169 
related to it ; the larval stage would seem, therefore, to be more im- 
portant than the adult ; at all events it is more thoroughly protected, 
and is the first to be adapted to suit its surroundings. The larvae of 
beetles are much more diversified than the adults. 
Although habits and conditions that surround the larvae have been 
very important features in the production of the present larval forms, 
some other force — which is undoubtedly heredity — has been at work 
in retaining them. The characteristics used for classifying adults 
cannot be used for the larvae— thus the antennae have an almost uniform 
shape, and the mouth-parts seem to have but little value ; no similarity 
can be traced between the mouth-parts of any particular family of larvae 
and those of the adults of the same family, but the mouth-parts of all 
beetles are more like those of adult beetles than they are like those of 
any other order of Insects. This is probably a case of precocious 
inheritance. In beetle larvae there are numerous cases in which a 
similar larval type has been independently acquired in two or more 
families. Some of the above generalizations will probably be found to 
be applicable to all orders of insects, while others are peculiar to 
beetles. 
Structure of Retina of Blowfly.* — Prof. B. T. Lowne returns to a 
subject on which there has already been much discussion, and criticizes 
particularly the memoir of Dr. S. J. Hickson, whom he accuses of 
making “an egregious misstatement of Dr. Weismann’s nomenclature.” 
He points out how Dr. Hickson and M. Carriere disagree, and allows 
that the former is right when he says that the nuclei of Carriere are not 
cells ; they are developed from cells, and each consists of a bundle of 
fusiform rods. Hickson’s nervous elements are “undoubtedly fine 
tracheal tubes,” and his “ neurospongium ” or terminal anastomosis, 
which is inadmissible on physiological grounds, is no nerve-plexus at all, 
but the tracheal plexus, the sustentacular framework of Prof. Lowne’s 
“ retina.” The author states that if the optic nerve be traced, its fibres 
are observed to run in larger or smaller bundles, invested in a very 
transparent sheath ; they terminate in the palisade layer by entering the 
fusiform elements. The sheath is continued over these last, and 
terminates on the inner surface of the basilar membrane. The tracheal 
vessels ultimately pierce this membrane, and run between the great rods. 
Prof. Lowne states that in size and structure the elements of the 
retina of insects are almost identical with those of vertebrates ; the optic 
nerve terminates in the protoplasmic inner segment, while the outer is 
transparent, resists stains, exhibits longitudinal stride, and swells up with 
water in both groups. In both it is easily destroyed, and frequently 
exhibits vacuolation. One difficulty in accepting the author’s views has 
been the structure of the great rods, and he owns that their appearance 
is in many sections perplexing. In life they are hollow tubes filled and 
distended with fluid ; in bad preparations they appear to be stellate in 
transverse section, and present no central cavity; in radial sections 
they are separated from each other by wide spaces which are often filled 
by distended tracheal vessels. 
The results of a long research are to confirm in the main the 
* Jouru. Linn. Soc., xx. (1889) pp, 436-17 (1 pi.). 
