1925] A Specimen of Melanoplus differentialis Thomas 289 
directions from the type. In organisms having bilateral sym- 
metry where either paired structures, or the two halves of un- 
paired structures standing on the middle line, are geometrically 
related to each other as optical images with reference to the meson 
or axis of major symmetry, meristic variation in such a bilateral 
series may take either one of two directions. Bilaterally paired 
structures may be placed farther apart, or they may be brought 
closer together; and in the latter direction the degree of variation 
may extend to their becoming united to form a single median 
structure or even to their complete suppression. Normally 
unpaired, median structures may on the one hand be divided 
into two, so as to form a pair of bilaterally symmetrical struc- 
tures, while on the other hand they may be reduced in size or 
entirely lost. 
Indeed, it contrasts with the variation resulting in the divi- 
sion of the median ocellus, examples are also available to illustrate 
meristic variation of the same bilaterally symmetrical series fo 
structures in the direction of fusion, and even to the point of 
complete suppression. 
Stannius 13 reported a worker honey bee specimen in which 
there is a complete fusion of the two compound eyes into a single 
symmetrical, elongate compound eye, that is uniformly con- 
tinuous across the middle line. In this specimen there is also a 
reduction from the three ocelli which are typical for this insect, 
to a single ocellus located on the dorso-meson. Lucas 14 reported 
the similar fusion of the compound eyes in a honey bee of doubtful 
caste, accompanied by the complete suppression of the ocelli. 
The median union of the ocelli could scarcely be interpreted 
as a reversion to an ancestral condition, and certainly the two 
opposite variations exhibited by these structures could not both 
be so interpreted. What may be the causes leading to either of 
these discontinuous changes we do not know; but it is not un- 
likely that variation in either direction may be found to result 
from the operation of the same factor or group of factors. At 
least it is better to refrain from drawing superficially plausible 
conclusions until we know more of the directive mechanism which 
13 Stannius, Mueller’s Archiv. f. Anat. u. Physiol., 1835, P. 297. PI. 
14 Lucas, H., Ann. Soc. Ent. France, 1868, ser. 4, v. VIII, p. 737, PI. 
