94 PROGRESS OF MICROSCOPICAL SCIENCE. 
insists that it should be done from a study of the winged adult or 
tmago, ‘ since a priori we cannot know how far the form of the larva 
is original or secondary.’ Other authors have with better reasons 
derived the ancestral form from the larva. 
“ Mayer’s ancestral insect, then, which he calls Protentomon, had 
a body divided into a head, thorax, and abdomen, the latter consisting 
of eleven segments, while there were six thoracic feet with five- 
jointed tarsi, and two pairs of wings, nine (and perhaps eleven) pairs 
of stigmata, a pair of salivary glands, and four excretory organs or 
Malpighian vessels, besides a well-developed nervous system, heart, 
and an aorta, as usual in existing insects. 
“ This hypothetical Protentomon is derived by Mayer from the 
worms,* in opposition to the suggestions of Fritz Miller and Brauer 
that the insects originated from the Crustacea. This worm, the 
parent of the half a million species of insects which have peopled 
the globe during the present and past ages, was ‘an unjointed worm, 
a common starting-point for the Tracheata and higher worms, and 
also a near relation of the ancestral form of the Crustacea.’ This 
worm then (1) transformed into a higher organism, with eighteen 
joints to its body and at least fourteen pairs of segmental organs, 
with perhaps also a masticatory apparatus in the form of jaws; and 
was perhaps nearly related to the existing Annelids. (2) A third 
step towards the insects was a form similar to the second, but with 
ventral and perhaps also dorsal appendages on all the segments; it 
was still aquatic. It transformed (3) into a worm with trachee and 
with dissimilar segments (the appendages in part beginning to dis- 
appear). It lived in fresh water, and is called by our author Pro- 
totracheas. (4) This Prototracheas became an Archentomon, still 
aquatic, with six feet, and clearly defined head, thorax, and abdomen. 
Finally, this fifth form acquired two pairs of wings, was terrestrial in 
its habits, and became (5) a Protentomon. 
“The author then discusses the ancestry of the different orders 
of insects. It is noticeable that in treating of them he begins with 
the Hymenoptera and ends with the Neuroptera, following in fact, 
unconsciously, the reviewer's classification proposed in 1863. The 
Linnean Neuroptera are, however, broken up into several orders, the 
author following the usual German system; but Mayer is the first 
German author, so far as we are aware, who places the Hymenoptera 
at the head of the insects, and the Celeoptera in the neighbour- 
hood of the Hemiptera and Orthoptera, where they unquestionably 
belong. 
‘“‘ Mayer adopts the suggestions of Biitschli and Semper that the 
air-tubes of insects originated from the segmental organs of worms, 
and, discarding Gegenbaur’s view that the air-tubes were at first 
internal, closed air-sacs, he believes that the stigmata or breathing 
holes were the first to be formed. It may be objected that as insects 
are already provided with renal vessels, it is not necessary to suppose 
* This view was advocated by Mr. Packard (though Mayer does not mention 
it) in ‘Our Common Insects,’ chap. xiii., entitled ‘‘ Ancestry of Insects ” (1878). 
This is the more inexcusable since Dr. Mayer quotes from the essay. 
