SOME PARASITES OF SIMULIUM LARV.E. 



The form and size of the imaginal discs, or histobla-t-. will 

 here be described in detail as they are of especial interest in 

 connection with the parasites. 



The thoracic hi-toblasts are twelve in number and aiv very 

 large and conspicuous in normal larva? as they approach lull 

 growth i Plate I., Fig. _j . 



The six discs to be found on each side of the thorax are the 

 following: 



1. Adult prothoracic leg histoblast situated at the base of 

 tin larval prothoracic leg. 



2. Adult mesothoracic leg histoblast -ituated ventro-laterally 

 on tin- me.-oihora< i'- -e-ment. 



.v Adult metathoracic leg histoblasl situated ventro-laterally 

 on the metathorai j, . m . 



4. Adult \\iiu hi-lobla-t - -imated dor-o-laterally oil the 

 mesothoracic segment Tin- in th- inatun- lar\a i- by far the 

 largest di-c and it - m COmes in contact with the nie-othoracic 

 ICL; <li 



5. Adult halteric hi-N-ltla-t This in the early -ta^es is 

 alino-a ,i~ large as tin- u in; disc, but it- 'vM'iiu th ami de\ elopment 



are verj -\<>\\ an-1 ii soon disappears under the rapidly expanding 



\\ ini; di-f. 



6. I'upal iv-pirati>ry tutt hi-t< >l >la-t . Tlii- i- -it n.ttfd mi the 

 pmtliorax and in ii- VOUl ha- the ap|earanee !' bein^ 

 quite lioiimli'^i'ii- \\ith a pn >t In -raeie \\in.u. It, h>\\e\er. -n>n 

 begins t> lake on .1 ddinin- t'..rni, and the r<miparati\ ely 

 traeheal tube- ran In- BCCn ^i"\\in- and leii-t helling 



the traii-pai'ent cuticle till tlle\ berime e. tiled tip a- indicated in 

 I'late I., Fig. 4. 



It -liquid be noted that tin- hi-tobla-t i- imt in the true sense 

 of the \\onl an "inia^inal" di-c since the re-pirator\- tilanieiiis 

 an- e\clu-i\ely pupal Organs, and ha\ e no e(|iii\al'-nt -tructnre 

 in any adult. Their -imilariu lo a pri honn ic \\ in- de-titute 

 of the \\ ini; meinbrane is of intere-t . A le\\ day.- before pupation 

 tin- "|>upal" hi-!obla-t becin- to darken. ri-nieiiiatioii com- 

 mence- at the apex of the folded tube- and slowly work- back- 

 ward- to\\ard tin- ba-e \\hen the di-c lake- on the appearance 

 shown in Plate lib. I . Later, immediately prior to pn- 



