The First Abdominal Segment of Embryo Insects. 
115 
B. The Sense-organ Hypothesis. 
Patten [’84] and Cholodkovsky [’89] are the only investigators, who have 
maintained that the pleuropodia of embryo insects may have functioned as 
sense organs. The following facts make for the probability of this supposi¬ 
tion: 
1. The pleuropodia are composed of peculiarly modified ectoderm 
cells. 
2. They roughly resemble such sensory structures as the Arthropod 
eye. 
3. Paired sense-organs are known to occur on the abdominal segments 
of many Arthropods. Cases in point are the curious Euphausia with its 
pairs of eye-like sense-organs and many Orthoptera that have sensory ana 
stylets. 
4. In Acilius , as Dr. Patten informs me, the small rods secreted by the 
pleuropodial cells are comparable to the retinal rods in the larval eye of 
the same insect. That the secretion in Cicada and Meloe flows together 
into one glairy mass, instead of forming bodies of like and definite shape 
capping the ends of the individual cells.may be due to the fact, that the 
organs are now merely rudimental structures. Dr. Patten tells me 
that the large lateral sense-organs of the embryo Limulus polyphemus 
produce a glairy secretion very similar to what I have described in Cicada 
[’89 a and b]. 
5. The pleuropodia are similar in shape and manner of development to 
the halteres of the Di tera and the pectinate appendages on the second 
abdominal segment of Scorpions, both of which modified appendages, as 
we have good evidence for believing, are functional sense-organs. 
These facts are weighty. It seems to me, however, that the supposition 
that the pleuropodia are sense-organs, is untenable, for the reason that no 
investigator has yet observed even a trace of nervous tissue in connection 
with the pleuropodial cells. In sense-organs as large as the pleuropodia we 
should certainly expect to find a well-developed neural element, since in 
the case of much smaller and more insignificant sense-organs it is not dif¬ 
ficult to detect the nervous connection. Granting that the pleuropodia are 
rudimental structures, it remains none the less improbable that a nervous 
connection, which in so large an organ must have been prominently devel¬ 
oped at one time, could have disappeared so completely while the sensory 
cells themselves underwent comparatively little diminution in size. The 
pleuropodia are, moreover, most conveniently located for innervation from 
the large ganglion of the first abdominal segment or from one of its main 
branches. 
It is, of course, possible, that in some or all forms the organs under con¬ 
sideration may have had a sensory function; but the facts accumulated up 
to the present, do not permit us to assign to the cells any other than a 
secretory function. 
